
Hi Dr. Lavelle, If we completed all the three quizzes on the purple workbook and received a higher score than on one of our quizzes, does our TA automatically change our grade for our lowest quiz score? I checked on MyUcla and it still has my old low quiz score. Thank you!
Hi Aldana, Yes it is automatic. Email your TA.
Professor, Assume we are titrating a weak acid with a strong base. Now, assume we have reached the stoichiometric point. Now, add, for example, 5 ml more of the strong base to the weak acid AFTER we have already reached the stoichiometric point. How would we calculate the pH of this type of solution after the titration? Would we simply assume the weak acid and its conjugate base no longer have much of an effect on the solution, so we consider only the concentration of OH and do pOH =  log [OH]? or Would we use the conjugate base formula of the weak acid?
Past equivalence point just consider the excess OH that you have in the system and calculate the pOH directly from that as you have mentioned. Don't forget to have the appropriate solution volume when doing so in order to have the correct concentration.
Hi Professor! I'm having a bit of a problem with the answers on the past final exams in the course reader. The answers in each step leading up to the final answers are almost always rounded incorrectly in terms of sig figs, which in turn results in a different answer than the one's I am getting. Would you please mind specifying how you want us to round the answers on the final? I've been told time and time again to round at the end, but clearly that's not the case according to the answers given in past finals. Thank you!
You should stick to not rounding your numbers in the calculations until the end. Any discrepancies in the answers will be small and not considered incorrect, even under a small amount of rounding.
Hello. For the 2011 final number 8, 0.100 M NH3 is titrated by 0.150 M HCl. I understand how to determine the stoichiometric point and the volume of titrant that must be added to reach halfway to the stoichiometric point (in the problem it is 5 mL). Why can the pH of the solution halfway to the stoichiometric point be determined by pH = pKa(NH4+) as described by the course reader?
Its always possible to go between pKa and pKb, pH and pOH values of a given system. Therefore, if you wish to comment on the pH of the system the pH HendersonHasselbalch equation must be used. At half way point your amount of base and conjugate acid are equal and therefore your pH = pKa of the conjugate acid.
Hello professor, I am confused with the practice final fall 2008 question 8. To me it seems that you can perform the same method for both parts, but I also understand that you can use the Henderson formula when you have a buffer. How do you know when there is a buffer in the problem, specifically this one? Is it in the equation that we would have to create? I would think you could use the Henderson formula for part A because there is a salt in the molecular formula, where you are not even given a chemical formula for Novocaine in part B.. . Thanks, Madison
If you are uncertain whether the Henderson equation can be used an alternative would be to set up an ICE table. This procedure would take longer but will guarantee a correct outcome if done properly.
Hi Professor Lavelle, In the answered question, you said the rule for pH: If you take the negative log of a value that has 3 significant figures, then your value for pH will go 3 numbers past the decimal place (for example 12.123). I want to make sure that you just simply go 3 numbers past the decimal place or you have to have 3 sfs in decimal portion. i.e If you take the negative log of a value that has 3 significant figures, the answer should be 12.012 (3 numbers past the decimal place) or 12.0123 (3 sfs past the decimal place) Could you please clarify it? Thanks
12.123 has 5 significant figures and 3 decimal places. 12.0123 has 6 significant and 4 decimal places. It is incorrect to take numbers apart and refer to individual portions as having their own significant figures. Here is a good example: The log of a product is equal to the sum of the logs of each multiplier: log(2.73 ×10^–5) = log(2.73) + log(10^–5) log(2.73) = 0.436: the answer has three significant figures, reflecting the possible error in the last digit of 2.73. log(10^–5) = –5.000000000...: the answer has an infinite number of significant digits because 10^–5 is an exact number and has no error. Then, log(2.73×10^–5) = log(2.73) + log(10^–5) = 0.436 + (–5.00000000...) requires that we use the rules for significant digits for sums, i.e., we can only add to the same decimal place as the value with the least significance. In this case, that is the third decimal place from 0.436, so 0.436 + (–5.000) = –4.564 or log(2.73 ×10^–5) = –4.564
For 5B on Fall 2008, your final answer was in units of atm because you used the form of the gas constant with atm. Since no gas pressure units were given in the problem, would be acceptable to use the gas constant in bars and find the partial pressures in bars?
For this course, if not specified, for pressure calculations use units of atmospheres.
Hi Professor! I had a question regarding problem 12.23 in the textbook. It asks to sketch the titration curve for the titration of 5.00 ml 0.010 M NaOH(aq) with 0.005 M HCl(aq). How is the final pH of the titration calculated?
See previously answered question.
Question 6B on Final 2010 says N2O4 > 2NO2 is an endothermic reaction since the covalent bond between the 2 nitrogen atoms is broken. Could you please explain this reasoning? Does this mean that if a covalent/ionic bond was created the reaction would be exothermic? Thanks
To break a covalent bond a certain amount of energy would need to be absorbed to do so. The opposite would be true as well, if you were to form a covalent bond some energy would be released.
Hi Dr. Lavelle In the first step of number 7 from the 2008 Final, why is it that the moles of HCl are subtracted from the moles of NH3 to get the moles of NH3, but the same thing is not done for finding the moles of HCl?
HCl is your titrant and you know its concentration and the molar quantity based on volume. NH3 is a weak base which is being titrated and you must consider to which extent it would be protonanted by the strong acid before setting up your calculations.
Hi Professor, When we are naming coordination compounds and the number of one of the ligands is one, do we have to add the prefix "MONO"? e.g. Ni(OH)(OH2)5 why is the naming of this compound pentaaquahydroxonickel (3) ion, but not pentaaquaMONOhydroxonicket (3) ion? Thank you!
If you have only a single type of a ligand then the name is written out without mono as prefix. You will not see a prefix for any singular quantity of ligand.
Professor, the last question i asked is a homework problem. P50912.9(a). The solution manual uses the ice box to calculate the pH, but can I use the Hasselbalch equation since HCN is a weak acid?
If you do this problem using the HendersonHasselbalch equation your answer will be pH = 9.46, which is exactly the same answer if you were to use the ICE table.
Hi Dr. Lavelle. I have a question from practice final 2008 (question 4A). it asks toname the following compounds: 1. [Co(OH)2(NH3)4]Cl For this one I need help with the oxidation number of cobalt because I got a different number. I did x2+0=1 but this gives me 1, while the correct answer is 3. 2. K2[PtBr4] I am just confused about why this is potassium tetrabromoplatinATE(III). Why is this an anion if there is no negative charge around the whole thing? Thank you!
1. [Co(OH)2(NH3)4]Cl can be separated into [Co(OH)2(NH3)4]+ and Cl. Therefore, your equation for the oxidation state of the metal should be x2+0=1 with x=+3. 2. We can do the same here and separate the ions into 2K+ and [PtBr4]2. This way you can see that the coordination complex is an anion and must have ATE ending.
Hello Professor Lavelle! I have a question regarding Question 6A on the Fall 2006 Midterm. The question states: The equilibrium constant Kc= 1.1 X 10^(2) for the reaction PCl5(g)>PCl3(g) + Cl2(g) at 400 K. Given that 1.0 g PCl5 is placed in a 250. ml reaction vessel, determine the molar concentrations in the mixture at equilibrium. I set up an ICE table and the equation for Kc. Since the quadratic equation that needs to be solved in this case is x^2+ 1.1 *10^(2)  2.1 *10 ^(4) =0, shouldn't the number in the denominator be 2 rather than 2.1 (as stated in the solutions)? Since the denominator of the quadratic is 2a which is 2(1)=2? Thank you very much!
The quadratic equation does have 2a in the denominator and so it should have 2(1)=2 for that quadratic equation which is posted in your question.
Hi Professor. Why can't I use Henderson Hasselbalch Equation for this question: Calculate the pH of the solution that results from mixing 30mL of 0.05M HCN(aq) with 70 mL 0.03M NaCN(aq)? What are the preconditions of using this equation? Thank you.
Can you be more specific. Is this a problem out of the textbook?
Hi, I was wondering what kind of questions we will be asked about buffers. Are titration problems buffer problems? Also, can we always use the ICE box method instead of using the HendersonHasselbach equation? Thank you.
The HendersonHasselbach equation can be used to determine the pH of a buffer solution. The same can be done using the ICE table, but can be more time consuming.
Professor Lavelle, One quick question, for 5.045, how many sfs are after decimal point? 2 or 3? Thanks
You should be careful about you wording. The number 5.045 has 4 significant figures and 3 decimal places. If you were to apply a log function to this number the final answer will have as many significant figures as the original value's decimal places and in this case is 3sfs.
Hi Professor Lavelle, Two questions about sfs. 1) If 0.15M*15.0*10^3 is only one of the steps to get my final answer, do I need to round off according to multiplication rule? I mean can I leave the answer as 2.25*10^3 or it has to be 2.2*10^3? 2) If I leave more digits for the value I put into negative log in order to reduce the error, how many sfs I should have after decimal place? eg if I know the sfs of the answer has to be 2 according to given concentration, but I put log 1.23*10^3, the pH should be 2.910 or 2.91. Or I should put log 1.2*10^3 instead of log 1.23*10^3? Thanks
1) Under the multiplication rule if you start with 2sfs and 3sfs then your answer must also be 2sfs. 2) Determine what number of significant figures your final answer must be in and round the final answer. Don't use values rounded to appropriate number of significant figures when it comes to intermediate calculation steps or your answers might be off.
Hi Professor Lavelle, Can I simply conclude that in multistep calculations, the sfs of the final answer is decided by the least sfs I used in all steps, no matter which step it is? eg. I used Ka(2sfs) and [base] (3sfs)to solve x which stands for [H3O+] and use the result to decide pH value. So the content I put in log should be 2sfs, right?
Don't make any assumptions. You must know how significant figures are affected by addition and subtraction, multiplication and division as well as log functions. Simply assuming 2 significant figures could be incorrect and would depend on what sort of work you had to carry out on those starting significant figures before applying the log function. ***************REVIEW ANNOUNCEMENT******************** The Saturday, December 8, 12:00pm Final Exam Review will be held in WGYOUNG CS76! Not in Franz 1178 as previously mentioned.
Hi Professor Lavelle, For partial pressure, what unit I should use? atm or bar? I think in the course reader you said "bar" is perfect; however, in 2008 FinalQ5B, you used atm. So I'm confused. Could you please specify it for me? Thanks!
The units of pressure you use will depend on the way the question is written. If pressures are given in bar, you can use bar. If the pressures are written in atm, as are many questions in your textbook, then use atm.
Hello, I was wondering, 1. Does it matter what order we write our coordination compound in. For ex. if I am given the name, does it matter order if I do [Cr(OH)(NH2)] or {Cr(NH2)(OH)]? 2. In some problems, information is given like 60. ml. Does that decimal point make it 2 sig figs or 1 sig fig still. Should I ignore the decimal, in terms of sig fig, after the whole number if there is no 0 after the decimal?
1. When you are writing out the chemical formula, you must write the ligands in alphabetical order. When you are changing from the written name to the chemical formula, there is not a set rule. A good rule of thumb, however, is to include the ligand with the greatest number first. For example, if you have 4 Br ligands and 2 H2O ligands, you would write the Br ligands first in the chemical formula. 2. If the value is written at 60. mL, that means you have 2 significant figures. 60.0 would have 3 significant figures, while 60 has only 1 significant figure. Be sure to go over the rules for significant figures in the fundamentals portion of your textbook.
Hello, I was wondering if there is a set sig fig for pH? For all the problems in the final, regardless of the sig fig in the problem, the pH has always been 3 sig figs.
To determine the number of significant figures in pH (and in any value you take the negative log of) there is a different rule that applies. If you take the negative log of a value that has 3 significant figures, then your value for pH will go 3 numbers past the decimal place (for example 12.123). So this value may have more significant figures, but the important part is how many digits past the decimal place you include.
I have a question about Q6C on Fall 2011 Exam: Consider the following reaction: 2HI yields H2+I2 (all in gaseous states) At 298K, Kc = 1.3x10^3, whereas at 783K, Kc=2.2x10^2. How do you determine if the reaction is endothermic or exothermic?
You can infer whether the reaction is endothermic or exothermic by looking at how the value of K changes with an increase in temperature. If the value of K becomes larger (as it does in this reaction), that means that more products are being formed at higher temperatures. If you increase the temperature and more reactants form, that means that the reaction is endothermic.
Hello Mr. Lavelle, As I was rereading the course reader I came across several questions. The first question I came across was what exceptions to the electron configurations do we need to know, for example Cr and Mo? My second question is related to bond order...does a bigger bond order mean a stronger bond thus a shorter bond length? My last questions are regarding drawing pH curves, what steps do we need to draw a pH? Also how do we calculate the pH at the halfway point to the stoichiometric point and the pH/volume at the end of the curve? Thank you
You should know the electron configurations for all of the first row transition metals. A larger bond order means a stronger and shorter bond. A double bond is shorter (and stronger) than a single bond. To sketch a titration curve, you need at least 3 points: the initial pH, the pH at the equivalence point, and one other point along the curve. The pH at the half equivalence point is equal to the pKa if you're titrating a weak acid with a strong base. To find the pH and volume of titrant needed at the endpoint of the titration (also called the stoichiometric point), you know that the number of moles of acid is equal to the number of moles of base. If you know the concentration and the number of moles, you can figure out the volume needed.
Consider the titration of 100.0 mL of 0.200 M acetic acid (Ka=1.8 x 10^5) by 0.100 M KOH. Calculate the pH of the resulting solution after 200.0 mL of KOH have been added. After 250.0 mL?
When solving titration problems, there are really 2 reactions that occur. The first reaction will be the neutralization of the acid and the base. This is like a limiting reagent problem. You will need to determine which species are in excess after the neutralization  those are the species that will contribute to the pH of the solution. You'll set up an equilibrium expression to find the pH.
Hi Professor Lavelle, I still don't understand under what conditions there is a buffer solution. I think as long as there is a weak acid/base, it can be considered as a buffer.In this case, the titration between a weak base and a strong acid can also be solved by pH=pKa+log([base]/[acid]. Could you please correct me? I used both ICE box and pH=pKa+log([base]/[acid])to solve a same problem. The answersn are pretty close, so I don't know which one I should use. Thanks.
Hi Eve, The ICE box method is explicitly solving for X (which is H3O+ or OH). This method is more accurate and must be used in titrations. The equation, pH = pKa + log([base]/[acid], is an approximation and is okay to use for buffers where [base] = [initial salt] and [acid] = [initial acid].
Hi Professor Lavelle, For question 12.19 (c) why the answer is CH2ClCOOH/NaCH2ClCO2 instead of CH3CH(OH)COOH/CH3CH(OH)COONa? The pKa of the former is 2.85 while the pKa of the latter is 3.08. Please explain it. Thanks
Hi Jingqi, The question asks to choose buffer components for a buffer at pH 3, since both are within +/ 1 of the pKa they are both suitable.
Do we need to know how to write compound reactions? Ex: Al2O3+ NaOH + H20 = Na[Al(OH)4] or just the structure and naming of compounds?
Hi Arev, You should know combustion reactions, acid/base, reactions, etc. (the ones discussed in class).
What value of Kc is considered small enough to make the assumption that X is negligible?
It is not a strict rule about the value of K, it depends also on the concentrations of acid or base you have in solution. There are several worked example of this in your course reader, for example on page 143.
Hi Professor Lavelle, For Final 2007, question Q4C, I don't understand why tha sfs for final answers for equilibrium composition are different. [PCl5]and [PCl3]have 3sfs but [Cl2]has 2sfs. You said we only have to round off at the final answer, but why they have different sfs? The given Kc has 2 sfs and the given concentrations have 3 sfs. I don't know which one should decide the sfs for my final answer. Could you please specify it for me? Thanks
The only value that will be evaluated for the correct number of significant figures is the final answer. More significant figures are included in the worked solution of this problem, but the final answer should have 2 sig figs.
For the final, do we have to explicitly show our work for the 5% rule in the equilibruim equations?
If you are making an assumption to remove x from parts of your equation you need to write that out. Be very clear in your answers if you want to get credit.
In the course reader's practice finals for the problems using K in equilibrium equations (ICE box method), it shows that we can eliminate/approximate x when K is less than or equal to a number times 10^4; is this still true for the final this year or do we only approximate when K is less than than 10^4?
It's is not strictly true that you can make that assumption for then K is less than 10^4. You must look at the difference between the concentrations you use and the value of K. If your concentrations are also on the order of 10^4 then the assumption will not be true.
In the 2008 final, question 8, part A asks to find the ratio of the concentrations of two species at a given pH. Part B asks to find the concentrations of Novocaine to its conjugate acid. In the worked out solutions, two different methods were used to compute both answers. Is there a specific instance when the HendersonHasselbalch equation or the equilibrium constant is used or are both methods sufficient to getting the answer?
The HendersonHasselbalch equation is specifically used when you have a buffer in solution. If you do not have a buffer, using this equation is not valid.
(Page 143 from the course reader) Calculating the pH of a salt solution After a couple calculations, we end up with x^2/(.15x)= 5.6 X 10^10 and we must solve for x. The notes say that if x is less than 5 percent of .15 then it is valid to approximate its value by setting it to 0 in the denominator. We then solve the equation: x^2/(.15) = 5.6 X 10^10 and end up with x = 9.2 X 10^6. My question is why is it mathematically valid to assume that x = 0 in one part of the calculation and then get a final xvalue other than 0? How could the final answer of x = 9.2 X 10^6 be founded on the prior assumption that x = 0?
The idea is that the value for x is so small, that the change it makes is very small, in fact so small that within the limits of significant figures it does not change the value. Let's look at that question as an example. If the concentration of NH4+ is 0.15x, and the value for x is 9.6e6, that means the concentration of ammonium ions is 0.15  0.0000096. Since x is so small, 0.15  0.0000096 is very close to 0.15.
When calculating pH in a strong acid weak base titration, if the amount of strong acid is beyond the stoichiometric point, can we just calculate the pH from the amount of strong acid left and ignore the salt produced? Thank you!
Once you pass the equilibrium point, the contribution of the salt to the pH of the resulting solution will be very small. To prove this to yourself, check to see how the pH would change. But yes, you can calculate the pH from the excess strong acid.
Should we know how to calculate the pH of acids/bases that require you to take the autoprotolysis of water into account? Thank you!
Since autoprotolysis was discussed during the lecture, you should understand how to perform those types of calculations.
Is hydrogen always attached to oxygen in an oxoacid?
There are several requirements for oxoacids. An oxoacid contains oxygen and at least one other element, and it has a bond between hydrogen and oxygen (this is your acidic proton). Thus molecules like sulfuric acid and nitric acid are oxoacids.
Hi, Professor. Can you work out the problem saying that for the reaction NO(g)>N2(g)+O2(g),if the initial partial pressure of NO(g) is 1.0bar and x is the equilibrium concentration of N2(g),what is the correct equilibrium relation? Why the correct answer should be K=x*2/(1.02x)*2? shouldn't we convert the partial pressure to the concentration? The problem is in 2006 final Q1C.
Remember that K can describe the equilibrium relationship in terms of concentration or in terms of pressures.
Hello. For the 2008 Final in the course reader, can you explain why is it that in 6E)there is graphite for the answer? Thank you!
You'll notice that if you only have a reaction involving CO2 and CO, you will not able to balance the reaction.
Professor Lavelle, How to decide the number of significant figures for pH value?
You should look up how significant figures are affected by logarithmic functions.
When writing the electron configuration for Br, should we just write Kr or would we have to write out the full configuration, Ar3d104s24p6
Write out the filled valence shell vs just Kr.
Hi professor. On the midterm exam, we were penalized for not putting the phases when writing out a reaction. In the practice finals, the phases were not required in the questions that asked for balanced equations. When will be be required to put phases when writing reactions?
It's always good practice to included phases when writing out chemical reactions, even when not asked for them.
Hi Professor Lavelle, I don't know when should I pay attention to sf. Can I conclude that i only need to round my final answers to correct number of sfs? And for any steps during the solution, i can have as many as sfs as I want? Please specify whether i need to in the following conditions. 1) If the question asks concentration at equilibrium, when solving x for concentration, do I need to round x to correct number of sfs? 2) If the question asks pH and I simply solve x for H3O+ concentration as one of the step, do I need to round x to correct number of sfs? 3) If the question asks concentration of either H3O+ or OH, when solving pH as one of the step, do I need to round pH value to correct number of sfs? Thanks!
Provide the answers that you are asked for with appropriate number of significant figures. While doing calculations you want as many decimal places as your calculator can store to attain the most accurate result. If the question requires you to use a number which you had to present with certain number of significant figures then use that number the way you have it in your calculator and not the rounded version for further work. Then once again present the final result of those calculations with the correct number of significant figures.
Hi Professor Lavelle, For question 12.5(a), I got pH=9.21 (9.31+log(0.060/0.075)) before I converted it to H3O+. I'm not sure whether I have correct number of sfs for my pH value, but i think since it's an addition, it's decimal places should be two. I checked solution manual, it sometimes uses ICE box and sometimes uses Hendersonhasselbalch equation in a same type of question. I'm confused. Could you please specify it for me? Thanks!
Don't use sig. figs. in calculations. Only show the answers you are asked for in sig. figs. Basically, don't use 9.21 to calculate [H3O+], use the value stored in your calculator such as 9.212894 and then round the outcome to fit the appropriate number of sig. figs.
Hi Professor Lavelle, For question 12.5 (a), if I use ICE box the answer is 6.1*10^10, but if I use Hendersonhasselbalch equation, the answer is 6.2*10^10. The answer on the solution manual is 6.1*10^10. Though two results are close but still different, so when doing buffer questions, which method I should use? Thanks
How much did you round your pH value before converting it to [H3O+]? Without severe rounding the answer comes out to 6.1*10^10 using the henderson hasselbalch equation.
When drawing a titration curve, how do you find the last point on the curve (question 12.23)? In other words, how do you find the final pH and volume of titrant added (for example, if you're given 0.005 liters of .010 M NaOH & .005 M HCL)?
You can arbitrarily choose the final volume of the titrant as long as you have a nice looking and complete titration graph as you seen in class.
Hi Professor Lavelle, I'm confused when you use the HendersonHasselback formula versus when you set up an ice box. For example in the homework for ch. 12 number 7 you can use the Henderson, but for number 5 you have to set up an ice box. what is the difference? Thanks, Madison
Ice box calculations are required when you wish to calculate changes in concentrations when equilibrium conditions are disturbed. Question 7 gives you concentrations of an acid and its conjugated base. There are no changes in equilibrium which are being forced upon this problem.
On page 469, # 11.19 (part d), the solutions manual states that Bi2O3 is basic rather than amphoteric. For what reason would Bi2O3 be basic and not amphoteric? Or is the solutions manual incorrect?
What is your source which states that Bi2O3 is amphoteric?
I have a question on acid strength trends. The course reader says that the longer the bond, the stronger the acid (so HI should be stronger than HF). However, it also says that more electronegative elements create stronger acids (so Hf should be stronger than HI). Which trend takes precedence?
You can look up the relative strengths of these acids to make a general comment. Otherwise, especially for this class, mixing different factors which contribute to acid strengths can get messy. Take it one concept at a time.
For Q8 in the 2008 final exam, you said "You made a substitution of pKa = pKw  pKb into your equation. You know that pH + pOH = pKw, now rearrange the equation to get the final answer." and when i rearrange it, i get phpKa=pKbpOH but the answer looks like it excluded the pOH since it's not accounted for. why?
You should not have a pKb term in your final answer. You've replaced pKb with pKw  pKa. Try doing it again and your pOH terms should cancel out. Remember you have another pOH term from log([OH]) in your problem.
Hi Professor Lavelle. One quick question  will there be video of Wednesday's review session uploaded or is there just audio?
Hi Marissa, My review session is available as both video and audio.
Hello Professor! I would like some clarification to this concept about calculating the pH of weak acids. Why is it that if the calculated [H3O+] is < 10^(7), then the solution is considered neutral? It says in the course reader that it is because of the result of autoprotolysis that there are 10^(7) M of H3O+. However, I can't make sense of it with this reason. Please help clarify it! Thank you!
If [H3O+] is smaller than 10^(7) then is it really making any significant contribution to the overall H3O+ concentration relative to the H3O+ concentration from the autoprotolysis of water. Due to this negligible contribution the pH can be considered neutral.
Can you explain Q8 on fall 2008 final exam. Why does pH=pka+log(N/HN+)?
You made a substitution of pKa = pKw  pKb into your equation. You know that pH + pOH = pKw, now rearrange the equation to get the final answer.
Why does aniline have 14 sigma bonds? I can only count 6 from the diagram given.
You are forgetting about the CH bonds since hydrogen atoms are not explicitly shown in the drawing. Also, do not forget the sigma bonds of the functional group on the benzene ring.
Hi! For final 2008, question 1B, once we find that the limiting reagent is Hg why do we multiply it by 1.5? Where does 1.5 come from? Also, for final 2009, question 2B, when writing the subshell notation for n=6, l=2 why is the answer 6d and not 5d (because the d is always one less than the row number)? Thank you! :)
For question 1B, the stoichiometric coefficient for AsF5 is 3 and Hg is 2. In order to do a direct comparison you must multiply by 3/2. For question 2B, (n) is the principle quantum number, if you wish to make a comment on which period this is the valence d orbital it would be the 7th (6+1).
Are Kp and Kc for gases used interchangeably because the ratio of the partial pressures of the products to those of the reactants is the same as the ratio of concentrations of products to reactants?
Kp and Kc are NOT used interchangeably. Kp does not equal Kc. There is an equation in your book that describes how to change Kp to Kc depending on the temperature of the reaction and the balanced chemical equation.
Hi Professor, I was trying to clarify the characteristics that make a an acid or base, weak or strong. In the Study Guide manual (pg SG134), it says that: "the conjugate base of a weak acid is a weak base" and "the conjugate acid of a weak base is a weak acid". Right below that it then says: "the stronger an acid, the weaker its conjugate base" and "the stronger a base, the weaker its conjugate acid." Could you please explain what it is saying, because to me it seems as if the statements are contradicting. Thank you!
There are only a few strong acids (there is a table in your book) and strong bases. Every other acid that is not one of those 7 strong acids is a weak acid. Every base that is not one of those strong bases (group I and II hydroxides) is a weak base.
Dr. Lavelle, on page 441 of the textbook it is said when looking at acid HA, the one with a more electronegative A is the stronger acid. But, HI is a stronger acid than HF and HCl for instance, but iodine has a lower electronegativity than fluorine or chlorine. So how come the books says otherwise? Thanks
HF is an exception to the rule. (Very rarely in chemistry can you say always or never because there are many exceptions.) HF is an acid of a halogen that isn't a strong acid, but the rest are. The rest follow that trend, however HF does not since it is not a strong acid.
Hello. For Fall 2006 Final Exam for Q8 B, can you explain why KBr is neutral and AlCl3 is acidic? Thank you!
Remember that acidity and basicity also applies to Lewis acids and bases. AlCl3 is a Lewis acid (there are only 3 bonds around the aluminum atom, so it can accept an electron pair). For KBr, when you add water it will become K+ and Br. When Br is in water, it could form HBr and OH. However, since HBr is a strong acid, that will dissociate back to H+ and Br, giving you Br + OH + H+, which will become Br + H20. Since it's just water that's formed, it will be neutral.
Hi professor Lavelle, Could you please specify how many sfs 250, 0.250 and 0.0250 have respectively? I'm confused. Thanks
For a value like 250, there are only 2 significant figures. If that same number is written at 250. then it has 3 sf, and if it's written as 250.0 it has 4 sf. The value 0.250 had 3 sf, and the value 0.0250 also has 3 sf. A good rule is to put the value you're given into scientific notation, that will help you to determine the correct number of sig figs.
For the 2007 Exam, 2B, is it wrong to write the electron configuration of I as [Xe}?
Yes, those are the same.
Hi professor can you explain how to solve this problem. A 0.500L reaction vessel at 700.K containes 1.20nmol SO2(g), 0.500mmol O2(g), and 0.10mmol SO3(g). At 700.K, Kc=1.7 times 10^6 for the equilibrium 2SO2(g)+O2> 2SO3(g). A.)Calculate the reaction quotient Qc Will more SO3 tend to form?
Remember that Q is different from K because you use initial concentrations instead of equilibrium concentrations. First you will need to calculate the concentrations of each species (you know the number of moles, and the size of the reaction vessel). Next, write an expression for Q, which means you need to make sure the equation is balanced first! Then you calculate Q using those initial concentrations. If K > Q, the reaction goes in the forward direction.
When do salts affect the pH of solutions and when do they not have an affect?
The effect of a salt on the pH totally depends on the salt. For example, something like sodium chloride will dissolve into sodium ions and chlorine ions, both of which are what are known as "spectator ions." However, if the salt you form is something like sodium acetate, the acetate ion is the conjugate base of acetic acid, and that will contribute to the pH.
Hi, How do you know that C6H5NH3+ is an acid instead of a base? Thank you
CC6H5NH3+ has a protonated amine, which indicates that this an acid. Since it is already protonated, it can lose that extra proton on the nitrogen to become C6H5NH2. This is the same reason that the ammonium ion, NH4+ is an acid while ammonia NH3 is a base.
Hi Professor Lavelle! This is regarding significant digits. I know that we are not supposed to round any number during calculation. We only round at the end. But when do you define "the end" of calculation? For example, let's say we are calculating the pH at the stoichiometric point. We first find the moles. Do you round at this point? Then if we were to do an ICE box, do we round the concentration numbers before we put them into the box? When we find the x value, do we round this number or do we keep as many digits as possible? Do you see my confusion? For some problems, I did not round at all and my answers were off from yours by one number. For example, I got 0.987 instead of 0.986. From your course reading, you did round at every step for some problems. I don't want to be penalized for not rounding throughout the problem. What should I do? THANK YOU!
Round the answers which you are asked for, but don't do it in your calculations. A difference between 0.987 and 0.986 is 0.001 which is essential negligible. Do not worry too much about very small discrepancies in answers such as these.
Hi Professor Lavelle! From Fall 2008 Final Exam Q5B, why are you using atm as pressure? Is it wrong to use bar? Here is the question: What are the equilibrium partial pressures of N204 and NO2 when 0.2 mol of N204 and 0.80 mol of N02 are sealed in a 2.00 L container at 100.0 degree C. When the reaction reaches equilibrium at 100 degree C, Kp = 11.0. THANK YOU!
1.00 atm is roughly 1.01 bar so there isn't much error in using either one. However as mentioned earlier in lecture use units of atm.
Hi Professor. Do we need to know how to solve for pH or pKa while considering autoprotolysis of water? One of the homework problems from Chapter 11 addresses autoprotolysis. Thanks.
Assigned homework problems are meant to help you comprehend the material. It's for your benefit to know how to do these problems.
Hi Professor, could you explicate more on amphoteric compounds. In the course reader, pg 129, it talks about Al2O3 being an amphoteric compound, but I am confused on how it acts as an acid and a base in the example given? Also, is it a Lewis acid/base or a Bronsted acid/base? Thanks!
it can react with acid to form AlCl3, it can also react with base to form AlO2. it's neither lewis acid/base
Hi Professor. For #75 on the homework (part c), the question asks you to calculate the pH of 0.055 M AlCl3 (aq). I do not understand why the chemical reaction for this solution would be Al(H2O)6^3 (aq) + H2O (l) <> H3O+ (aq) + Al(H2O)5OH^2+ (aq). Where did the H2O molecules inserted in the first reactant and last product come from? How do you know that AlCl3 is an acid in the first place?
Al corresponds to Al(OH)3 which is a weak base, Cl corresponds to HCl which is strong acid, that's why it's acidic. and Al tends to form those form of hydrolized compound
Hi Professor Lavelle. I am having trouble with #71 in the homework (parts e and f). The directions ask you to determine whether an aqueous solution of each salt has a pH equal to, less than, or greater than 7 (and write a chemical reaction to justify your answer). Part e gives you AlCl3. I'm having trouble understanding why this would be considered an acid (and hence have a pH less than 7). Part f gives you Cu(NO3)2 and I also don't understand why this is an acid. Would you be able to explain how you determine that each is an acid? Thank you.
for e, because Al corresponds to Al(OH)3 which is weak base while Cl corresponds to HCl which is strong acid. same reason for part f
Hi Professor Lavelle, According to 11.55, why CH3 has electrondonating properties? How about anions like Cl, O2,do they have electrondonating properties? Or they only have electronwithdrawing properties£¿ How do you know whether it has electronwithdrawing or electrondonating properties? Thanks
CH3 does have electrondonating properties because C has lone pair. those anions are electronwithdrawing cuz electronegativity.
Hi Professor Lavelle, I don't understand the solution for 11.121. First, why B(OH)3 cannot have resonance structure? I think it can have one double bond and the total valence electrons are still 24. Second, could you please explain the sentence "B(OH)3 is a very weak acid because it does not have a conjugate system to delocalize the electrons on the oxygen to weaken the OH bond"? Thanks
it doesn't need to have resonance structure, so resonance is not important. yeah, it's weak, there's no electronwithdrawing group or double bond to stabilize the deprotonate form
Hi Professor Lavelle Though there is a solution for 11.133, I still cannot understand it. I don't understand why electron delocalization has something to do with acidity. Why CH2 can cause greater delocalization? And why because adipic acid has stabilized carboxylic anions so that the two protons on the adipic acid can be easily deprotonated simultaneously? I think it's stable, it should be more difficult to deprontonate. Could you please explain more? Thanks
ch2 doen't cause strong delocalization. the reason that PKa1 of HOOCCOOH is smaller than that of the other one is because the electronwithdrawing effect of HOOCCOOH is stronger than the other one. So the HOOCCOOH acid is stronger. adipic acid is not easy to deprotonate compared to the other one, that's why it's PKa value are larger
For number 11.71 d), why is KBR neutral? For 11.71e), how are we supposed to derive Al(h20)6 3+ from AlCl3? Where does the H20 come from for e) and f) of 11.71? Also, for number 2.73) how are we supposed to calculate the pH ?
HBr and KOH are strong acid n base, that's why for e and f, it's the aqueous solution, so there's gonna be water existed. u mean 12.73?
For number 11.109, why is it that we must take into account the autoprotolysis of water when [H30+] <= 10^6?
because when [H+]<=10^6, the H+ produced by water is not negligible
For 12.23, how do we know that we need to add 30.0 mL of HCl and that the final point on the PH curve is 2.54?
30ml? it's not 30 ml to reach the stoichiometric point. the PH at stoichiometric point is 7
Hi Professor. How can you tell that KF is a base and that its pH value is <7, whereas KBr is a neutral compound and has a pH=7? Thank you.
See previously answered question.
For question 11.71, how can you tell that KBr is neutral, that AlCl3 is acidic, and that Cu(NO3)2 is acidic? Where does the H2O come from in the chemical equations for parts e and f?
Br is an anion of a strong acid and does not affect pH. Metal cations act as Lewis acids and increase [H3O+]. Don't forget that we are working in aqueous environments.
For questions 12.17 (c&d), 12.19 (b&d), and 12.21, how are we supposed to calculate the values of pKA2 without the concentrations?
These questions do not ask you to calculate the pKa values. You can answer these questions by looking up the pKa values either in your textbook or external sources for the compounds mentioned.
Hi Professor. How do you know that AlCl3 (obtained at the stoichiometric point in a titration) has a pH below 7? I'm having trouble breaking the salt into two ions of different acidic and basic strengths to determine the pH.
You know that small highly charged metal cations such as Al+3 act as lewis acids to increase [H3O]+ concentration.
Hello Dr. Lavelle, This isn't concept related, but I heard if we get wait listed for chem 14B we will still be guaranteed in the class, is this true? Thanks
Hi Shiva, Waitlisted students have a very high chance of getting in. See previous post.
What is we still can't get int chem 14B even on waitlist?
Hi David, There is no secret formula but to check on a regular basis to see if waitlist spots have opened. Typically each class has about 35 (~10%) students dropping as students change their schedule, etc. This results in about 100 students moving from the waitlist into the class, which means waitlist places will open up (maybe 12 each day for the next month). Biggest movement is during week 1.
Hello Professor, will Chem 14B be taught like how Chem 14A is being taught? That is, 3 quizzes, 1 midterm, webcast lectures, etc.?
Hi Alvin, Yes, very similar.
Hi professor. We discussed in class the the Ka1 of a reaction will always be larger than the Ka2. The book tends to say that the smaller the Ka, the weaker the acid. However, we see that the reaction has a lower pH after the second reaction in which we get the Ka2 from. I understand that the reason may be because more H3O+ ions are produced, but regardless of that, this situation contradicts the statement made before, (i.e the smaller the Ka value, the weaker the acid). Thank you!
Are we considering these deprotonations separately or in the same system? If they're in the same system then you need to consider the relative pH change after the first deprotonation versus the second. Otherwise, you are forgetting that the initial pH drop is from the dissociation of the first proton and the second pH drop occurs from the already reduced pH and not once again from a neutral solution.
Hi Professor Lavelle, for problem 11.27, how come the solutions manual sets [OH]=2[Ba(OH)2]? I thought it would be [OH]=[Ba(OH)2]/2
Remember how to calculate concentrations and dimensional analysis. If you start with the concentration of barium hydroxide, for every 1 mole of barium hydroxide, you have 2 moles of hydroxide ions, which is the species that contributes to the pH. If you set up your dimensional analysis correctly (make sure your terms are set up so they cancel appropriately), you should see.
Hi professor. We discussed in class the the Ka1 of a reaction will always be larger than the Ka2. The book tends to say that the smaller the Ka, the weaker the acid. However, we see that the reaction has a lower pH after the second reaction in which we get the Ka2 from. I understand that the reason may be because more H3O+ ions are produced, but regardless of that, this situation contradicts the statement made before, (i.e the smaller the Ka value, the weaker the acid). Thank you!
Can you be more specific with what reaction you're describing? Remember that essentially the Ka value is telling you information about how much dissociation there is in the acid. For a strong acid, the dissociation is complete. However, for a weak acid, the solution is in equilibrium because it does not completely dissociate. The larger the Ka value, the more dissociation there is, meaning that the acid is stronger (i.e. more dissociation means closer to a strong acid).
Hi Professor. I am confused as to when it is appropriate to use the HendersonHasselbalch equation versus log([H3O+]) to calculate pH. Would you be able to clarify when you are supposed to use each equation? Thank you.
The HendersonHasselbalch equation is the rearranged form of the dissociation constant equation. So regardless of how you approach a problem you still need to know [H3O+] and HendersonHasselbalch equation provides you with a convenient way of doing so in a single step.
Hi Professor, I have a question regarding acids and bases. I know that [pH + pOH = pKw] and that [pKa + pKb = pKw], so does that mean that pH the same as pKa and pOH is the same as pKb?
It is not true that pH = pKa or pOH = pKb except in one very specific instance. When you have added half of the volume needed to reach the stoichiometric point (the half equivalence or half stoichiometric point), then pH = pKa for that reaction. You cannot make that assumption at any other point along the titration curve.
When you have an atom such as Oxygen that has two lone pairs off it and is double bounded to an atom of Carbon, why is the bond composition of the lone pairs Osp^2? Thank you.
Remember that hybridization comes from regions of electron density. So the double bond to a carbon counts as one region, and each lone pair of electrons counts as one region, so there are three total regions of electron density around the oxygen, making the hybridization sp2.
Hi Professor. I am confused as to when it is appropriate to use the HendersonHasselbalch equation versus log([H3O+]) to calculate pH. Would you be able to clarify when you are supposed to use each equation? Thank you.
Taking the negative log of the concentration of hydronium ions is always a correct way to find the value of pH, since this is the definition of pH. Specifically when you are using a buffer, you can use the HendersonHasselbalch equation to determine the pH of the buffer you are forming.
Dr. Lavelle, Question on Example 12.1 in the textbook pg 477 "Suppose we are culturing bacteria that requires an acid environment and want to prepare a buffer close to pH = 4. We prepare a buffer solution that is 0.040 M NaCH3CO2(aq) and 0.080 M CH3COOH(aq) at 25oC. What is the pH of the solution?" Given Ka = 1.8 x 10^5 Why can we assume that the extent of protonation of the acetate ions and the deprotonation of acetic acid molecules is so small that the concentrations of both species are nearly the same as their initial values? Is it simply because the acids and bases are weak, or does the fact that the Ka is < 10^4 have anything to do with it? Or is the Ka < 10^4 because the acid is weak? Or are there any other factors contributing to this assumption?
There are two ways you could solve these types of questions. One is using HendersonHasselbalch equation, which is unique to buffers. Additionally, you could set up an iCe table and solve for the concentration of hydronium ions that way. If you use the second method, remember that you will have a value for both the acid and the conjugate base (so in other words your products won't have a value of zero).
Could you help explain how the salt KF has a pH above 7 and the salt KBr has a pH of 7? Thank you so much!
F is a conjugate base of a weak acid (HF) and Br is an anion of a strong acid (HBr). F would therefore affect the solution pH by forming HF to some extent and producing OH while Br cannot do the same. Think about the Ka values for HBr and HF and what effect it has on dissociation of each.
Hi Professor. Problem #21 in Ch.12 of the textbook asks, "What must be the ratio of the concentrations of CO3^2 and HCO3^ ions in a buffer solution having a pH of 11.0?" When solving the problem with the equation, pH=pKA + log(Co3^2/HCO3^1), why do you have to use the value of pKA2, and not pKA1? Thank you.
When looking at pKa values for acids that can lose multiple electrons, the first pKa is always going to be the fully protonated acid losing one proton. So in the case of carbonic acid, pKa 1 would be from the reaction of H2CO3 becoming (HCO3). Since the problem asks about the ratio of (HCO3) and (CO3)2, which comes from losing one more proton from the previous species, this will involve the second pKa value.
Hi Professor. When we are asked to write the formulas for the conjugate acid/base, how do we know where to remove the hydrogen atom? For example, if we are asked to write the conjugate base of C6H5OH, how do we know to remove the hydrogen from the oxygen and not the carbon?
Some of that will come from practice. You will notice that protons will not come from a carbon ring (something like C6H6) because that will create an unstable ion. Think about what sort of structure will form if you remove a proton, leaving behind an extra electron. Those extra electrons will typically prefer to reside on a more electronegative atom. Keep those in mind as you begin to work through the practice problems.
Hi Professor, when finding the pH for an aqueous solution like NH3, the book gives the chemical reaction NH3 + H2O > NH4+ + OH. My question is: Why can't the reaction be NH3 + H2O > NH2 + H3O+? Can you tell if the compound will be acidic, basic, or amphoteric just by looking at it? Thank you
You should start to identify compounds that are acids and bases, some of that will simply be memorization at the beginning. Since ammonia (NH3) is a base, it will accept a proton, thus the second reaction you wrote is incorrect.
Hi Professor, when setting up a chemical reaction equation, how do you know if you're supposed to use one arrow > as opposed to arrows going both ways <>? I noticed some of the answers to the recommended problems in Acids and Bases varied between the two types of arrows. Thank you
The two different arrows types describe two different types of chemical reactions. The single headed reaction arrow (>) means that the reaction goes to completion. In other words, it is not in equilibrium. This is the reaction arrow you will use for strong acids and strong bases. The doubleheaded arrow (<=>) is used to describe a reaction that is in chemical equilibrium. It's used for weak acids and weak bases, among other types of reactions. A good tip is that if you see a value for K (the equilibrium constant), that's a hint that you should use a doubleheaded arrow.
Hello Professor Lavelle, I have a question regarding Kc and Kp values and how to solve them. If I am given a reaction at equilibrium and its Kc value how do I find the Kp value. Thanks!
You will need to know the temperature of the reaction in order to change between the Kp and Kc value. To solve the problem, you must first balance the chemical reaction, then you can use the equation in your book: Kp = Kc(RT)^(delta)n.
Hi Professor Lavelle. When calculating pH levels, does it follow that the number of digits following the decimal point in a pH value be equal to the number of significant figures in the corresponding molar concentration in the original problem? I was wondering because for some of the homework problems (such as Ch. 11 #33, part A) the solutions don't follow this rule, which is confusing me. Thank you.
It depends on what you are doing. If you are determining the pH from a pOH, in that case you are using subtraction. If you are calculating the pH by taking the negative log of the concentration of protons, then you follow the logarithm rule. The logarithm rule (in simple terms) means that first you should identify the number of significant figures in the value for the concentration. Your answer should go to that decimal place. In other words, if taking the negative log of a value with 2 significant figures, your answer should go to the hundredths place (2 past the decimal place).
How professor. On the titration problems in the textbook, I end up rounding differently and using more digits than the solutions manual during calculations and I get slightly different answers from the answers given in the solutions manual. On the final, will i be penalized for missing the exact answer by a tiny bit?
If you round as you go through the problem, your answer is likely to be different from your textbook. A good rule of thumb is to keep all of the numbers in your calculator and only round at the very end.
Hi professor. During a strong acid strong base titration, how do we calculate the final pH? How do we find the volume of titration added for the final pH?
For a titration with a strong acid and a strong base, just as with any titration problem, you'll need to determine the resulting concentration of protons. For a strong base, you are effectively creating hydroxide ions (remember, since a strong base fully dissociates, the concentration of the base is the concentration of the hydroxide ions) and for a strong acid you get protons. For every 1 mole of protons and 1 mole of hydroxide ions, you will produce 1 mole of water, thus they will neutralize each other. Determine which you have in excess, acid or base, and calculate either the pH or the pOH from this resulting concentration. The volume will be the amount of acid you started with plus the volume of base you added (so you use the TOTAL volume when determining the final concentrations).
For 12.7, how do we know the starting pH is 1.92? The Ka value of HSO4 is not given in table 11.1 or table 11.2/ Thanks!
Ka value of HSO4 has nothing to do with the starting PH of H2SO4. it's the Ka of H2SO4 that matters.
Hi Professor Lavelle! This is from online module: The photosynthesis reaction, 6 CO2(g) + 6 H2O(l) ⇌ C6H12O6(aq) + 6 O2(g), is endothermic. What effect will the following changes have on the equilibrium composition. a) Water is added. Thank you.
no effect
Hey professor, if we are asked to write the Kc of an equilibrium equation do we include gas phase compounds in the expression. And what if we were asked to write the Kp, do we include aqueous substances in the expression. Thank You
yes, include gas no, no gas for Kp
For one of the questions in the book for Chapter 16, it says that for [CoBr(NH3)5]SO4 the charge on cobalt is (II). Isn't it (III) because sulfate has a charge of 2 and bromine has a 1 charge?
yes, it is III. the answer is wrong i think
Why is [Cr(F)3(I)(H2O)2] written out with an ion at the end? (question 5 on quiz 3 Fall 2011) The complex doesn't have an ion so why would we write it out with ion at the end?
Cr has oxidation number of 3. so the compound is actually 1 charged
Do we need to know the conversion between Pa and atm? If so, what is the conversion?
no
Hi Professor, I have 2 questions. 1) When solving for Kp, will we get deducted a point if we accidentally put it in the form of e.g. [HI]^2 / [H2][I2] instead of PHI^2 / PH2PI2? and 2) I always forget, when we divide by the molar mass of an atom, how many sig figs are we supposed to use? e.g. for C, should we use 12, 12.0, or 12.01? Isn't the basic rule to round off in the end for a final calculation/answer? Thank you.
1. it's recommended to put P in front of formulas 2.use the one in the periodic table. yes, round off the final answer
Hi Professor! I have a question regarding a HW problem from Chapter 10. For number 37b, why is the answer in square root? I was thinking that since every coefficient was halved (from N2 + 3H2 (equalibrium sign) 2NH3 to (1/2)N2 + (3/2)H2 (equilibrium sign) NH3), the answer must must be 41/2.
No, the K is not halved. u need to calculate the K again using the halved coefficient. it's gonna be a completely different number
When naming compounds, I understand that you need to put it in alphabetical order. Why is 16.29's compound name "sodium bisoxalato(diaqua)ferrate(III)" instead of "sodium diaquabisoxalatoferrate (III)?" I thought "aqua" comes before "oxalato" because we are supposed to ignore prefixes? Thanks, Theresa
that's because the aqua is not the ligand of the compound, it's included in the other ligand. see answer
Hi Professor! In the course reader it says to use mono, di, tri, tetra, and penta to indicate the number of ligands. In the next line it says to use the prefixes bis, tris, and tetrakis. How do you know what set of prefixes to use? Thank you!
like for the question 1 of the last preparation quiz. there's a ligand "en" with the name ethylenediammine. so there's already a "di" in its name. then we should use bis if there's 2 of this ligand
In the purple book one of the questions (pg 51 number 5) asks to write the formula for the coordination compounds. The compound is Diamminetetrahydroxochromate (II) ion and I am confused about the ordering of whether to first put the OH before the NH3 or vice versa. The answer says OH but the textbook says to arrange alphabetically by symbol letter, which means NH3 would be placed before the OH. Which is correct?
either is correct
Hello Professor. If an equilibrium question gives you partial pressures at equilibrium for the gases in a chemical reaction and tells you that half of one of the gases (let's say gas A) was removed, is it safe to say that the new partial pressure for gas A is 1/2 the original partial pressure given for gas A? If not, how would you solve for the new partial pressure for A? Thank you!
yes, correct
Hello Professor!! When writing the equilibrium expression of a reaction that consists of a gaseous chemical with a fraction as its coefficient in the equation (such as 3/2 H2 (g)), is it acceptable to write it [H2]^3/2 in K or is it proper to make it a whole number (such as [H2]^3) in K? Thank you so much!!!
in the fraction form is acceptable
Hello Professor! When we are to write the equilibrium constant for a reaction, such as Pb(NO3)2(aq) + 2KCl(aq) > PbCl2(s) + 2KNO3(aq), is it enough to write it K = [KNO3]squared/([Pb(NO3)2][KCl]squared), or do you want us to further simplify the reaction to the ionic state and write K as 1/([Pb2+][Cl]squared)? Or are both acceptable on the test? Thank you so much!!!
further simply it if u can
Hey Professor Lavelle, I was wondering if you could help me where to start on the last problem on the last practice quiz in the purple workbook. I'm not asking for an answer just a hint on where to begin with this problem. Thanks.
is the practice quiz the one that ur supposed to hand in? the very last quiz?
Hi, I'm really confused with regards to writing the chemical formula of coordination compounds. So alphabetical order of the ligands only matters when naming the compound NOT so much when writing the formula, correct? Because in the book, I thought it said to write ligands alphabetically with writing chemical formulas but the course reader and the solution manual don't seem to follow that.
yes
If you are given a formula in word form, is there a specific order for the actual chemical formula? (ex. Pentaamminebromocobalt (III) is written as [CoBr(NH3)5]2+, but can it be written as [Co(NH3)5Br]2+?]
either is correct
On the Fall 2010 Practice Quiz 3 question 2 asks for the formula for pentaaminebromocobalt(III) which I thought would be [CoBr3(NH3)5] but instead the answer key says [CoBr(NH3)5] 2+ but the question never specifies that its an ion, so which answer is correct?
the sequence of the ligands in the chemical formula doesn't really matter. either is correct as long as the naming follows a certain sequence
Hello Professor Lavelle, I'm having trouble with these two questions from the equilibrium chapter. 10.39) When 0.0172 mol HI is heated to 500.K in a 2.00L sealed container, the resulting equilibrium mixture contains 1.90g of HI. Calculate the K for the decomposition reaction 2HI(g)<=>H2(g)+I2(g). 10.41) A 25.0g sample of ammonium carbamate, NH4(NH2CO2), was placed in an evacuated 0.250L flask and kept at 25 degree C. At equilibrium, 17.4mg of CO2 was present. What is the value of Kc for the decomposition of ammonium carbamate into ammonia and carbon dioxide? The reaction is NH4(NH2CO2)(s)<=>2NH3(g)+CO2(g). THANK YOU!
39. 1.90g HI is the final gram of HI at equilibrium convert the gram to mole then u calculate the molar concentration of the initial concentration and equilibrium concentration. do the ice box, calculate the K 41. similar as above calculate the molar concentration of the initial reactant and equilibrium CO2. complete the ice box
Hi Professor. For question 8 on the final practice quiz in the workbook, it says the equilibrium partial pressures are 0.28 atm HI, 0.033 atm H2, and 0.045 atm I2 for the reaction H2(g)+I2(g)>2HI(g) and K for this reaction is 53. When it says half of HI is removed from the container, does this mean the partial pressure of HI is decreased by half of its original amount? What would the set up for the ICE box look like for this problem when it asks what is the new partial pressure for HI when it reestablishes equilibrium? Thanks.
yes, half partial pressure H2 + I2 > 2HI I 0.033 0.045 0.14 C x x 2x E 0.033x 0.045x 0.14+2x
Hi, Professor, what does it mean by saying "should one use a large or small container to produce most possible hydrogen gas from NaAlH4" for the question 5 on page 55, workbook. Does it mean if we change the volume of the system at equilibrium, to produce more H2 gas, should we compress it or expand it?
yes, your understanding is correct
Hi Professor Lavelle, For the willbegraded Quiz 3 in the WB questions 5 and 7 asks for the equilibrium expression. Is that equivalent to asking the equilibrium constant expression? Thank you!
yes
Hi professor. When calculating sig figs, can you have a result with less sig figs than the least given sig fig? For example, if the least given sig fig is 3 and you get an exact answer of 3.6, should you make it 3.60 to give it a 3 sig fig?
yes, make it 3 sig figs
Hi professor. When calculating sig figs, can you have a result with less sig figs than the least given sig fig? For example, if the least given sig fig is 3 and you get an exact answer of 3.6, should you make it 3.60 to give it a 3 sig fig?
yes, make it 3 sig fig
Dr. Lavelle, where do I find the online modules covering acids and bases?
Check VOH
Professor Lavelle, on page 105 of the course reader, where is the m^2 (denominator) derived from when looking at the equation of P. In other words, how do I know that the area is a squared variable? Moreover, what specific details should I focus on when reviewing the properties of gases, any specific equations or facts?
Hi, The unit of area is m^2. For example, the area for a rectangular is length (m) * width (m). Therefore, you can get the unit of area is equal to m^2. For the properties of gases, you need to know the idea gas law, the relationships between each variables, the definition of partial pressure, and the conversion of different unit of pressure. Best,
Hello. Why is CO3 2 (in problem 16.31b )considered both a mono and bidentate, and not just a bidentate? Thank you.
Hi, Because the CO32 ion can use either one or two oxygen atoms to bind to a metal ion. Best,
Hi Professor Lavelle, When we're asked to write the proton transfer equilibria for acids like H2SO4 and H2PO4, do we need to write all steps? Or only the next step? Also, for H2PO4, do we need to write H2PO4(aq)+H2O(l) >H3PO4(aq)+H3O+(aq) if the question indicates H2PO4 as an acid? Thanks
Hi, It is better for writing down all steps and this is the correct reaction for your example,H2PO4(aq)+H2O(l) >HPO4(aq)+H3O+(aq) . Best,
Hi professor Lavelle, for reaction HNO3(aq)+HPO42(aq)>NO3(aq)+H2PO4(aq), the Bronsted acid is only HNO3 or both HNO3 and H2PO4. The answer is only HNO3, but I think it should be HNO3 and H2PO4 because two sides react the same. Thus, in the reverse reaction, H2PO4 is the Bronsted. Also, conjugate acid and base should also have two answers instead of one. Am I right? Or we only need to consider the forward reaction? Thanks
Hi, For the Bronsted acid, you need to consider which one is proton donor. In this example, HNO3 is a proton donor, and its conjugated base is NO3. HPO42 is base and its conjugated acid is H2PO4. Best,
Hi, Professor! I have a question for 10.5 on textbook. In the solution, why the denominators of moles fractions are 17? Can you explain why it works the problem that way?
Hi, By the definition of partial pressure. partial pressure is equal to Xi (molar fraction) * total pressure. In the flask 3, there are 5 X2 and 12 X. Therefore, the molar fraction of X2 is 5/(12+5) and so on. Best,
Hi, Professor! I have a question for 10.5 on textbook. In the solution, why the denominators of moles fractions are 17? Can you explain why it works the problem that way?
Hi, By the definition of partial pressure. partial pressure is equal to Xi (molar fraction) * total pressure. In the flask 3, there are 5 X2 and 12 X. Therefore, the molar fraction of X2 is 5/(12+5) and so on. Best,
Hello, can you please explain number 10.5 in the textbook. part c. i do not understand the explanation in the solutions manual. thanks!
Hi, The decomposition is X2>2X, and you can find that the flask 3 represent the point of reaction equilibrium. You can count how many X2 and X in the flask. There are 5 X2 and 12 X in the flask. By the definition of partial pressure, you can write down partial pressure of X is (12/17)* total pressure and so on. Best,
Hi Professor, I have a question concerning number 69 on page 418 in the book. I was confused with the solutions, where they took the initial values to get a "new initial" value.
Hi, You can see that the Kc is extremely small (10^34). Therefore, you can image that this reaction is hard to move forward but easily push to the left side (reactant side). That's why you can set the new initial partial pressures and then calculate continuously.
Hi Professor Lavelle, For question 16.29 letter A it askes you to write the formula for potassium hexacyanidochromate (III). I thought that when the metal ended in ate it indicated a negative charge on the complex compound so I ended up with the answer K[Cr(CN)6]2 (indicating a 2 charge on the compound) however the answer in the back of the book is K3[Cr(CN)6]. How come there is not an overall negative charge on the compound? Thank you!
Hi, The coordination complex anion is [Cr(CN)6]^3. Because you know the oxidation number of chromate is +3 and there are 6 CN. Therefore, the total oxidation number of coordination complex anion is +3+6*(1)=3. Also, that is why there are 3 potassium elements in this coordination compound.
Hi, Professor. For "aqua" and "bisethylenediamine", which one should I write first? Also, on page 683 Toolbox 16.1, tip 6, why do they put "thiocyanato" before "pentaaqua"?
Hi, by alphabetical order, you should write down aqua first.
Hi professor. When writing formulas for coordination compounds, will it matter if we write ox or c204 and h2o or oh2? Is there a preference?
Hi, For C2O4, we would like to write down the ox. For H2O, just write down H2O.
Dr. Lavelle, on question 16.29 of the chemistry textbook I am given the name of the complex: potassium hexacyanidochromate (III) and expected to figure out the formula for this complex. The answer to the problem is K(sub3)[Cr(CN(sub6))]. I am confused as to why the roman numeral is set at three; I see that the chromium is linked to the cyanide and the potassium.
Hi, Because the anion is [Cr(CN)6]^3, for the neutral compound, the charge of cation should be +3. That is why there are 3 potassium in the compound.
In naming coordination compounds, you're supposed to change ide to ido. However, in question 16.27 a, which is name complex [Fe(CN)6]4, the proper answer is hexacyanoferrate (II), not hexacyanidoferrate (II). In 16.27 c, the same thing; [Co(CN)5(OH)2]2 is aquapentacyanocobaltate (III) ion, not aquapentacyanocobaltate (III) ion. But in 29 a, potassium hexacyanidochromate (III) is K3[Cr(CN)6]. Why are the ones in 27 cyano and 29 cyanido? Thanks Prof!
Hi, For a more systematic approach, the application of the ending ‘ido’ to all anionic ligands with ‘ide’ names in additive nomenclature. eg. cyanido and chlorido instead of chloro and cyano. Therefore, there's no difference between cyano and cyanido. You can use either one.
Hi professor. I'm having trouble with a problem from the work book. "Equilibrium partial pressures are 0.28 atm HI, 0.033 atm H2, and 0.045 atm I2, K=53." Is it assumed that K was calculated using the partial pressures in atm and not bar?
Hi, yes, you can check the unit of partial pressure to decide the unit of Kp. Best,
Hi Dr. Lavelle, Since classes have been cancelled for Wednesday (thank you!) I was wondering if office hours would be rescheduled, since they would be the last before the quiz for those in Wednesday morning discussions. Thanks again, Grace
Hi, The office hours are the same. Best,
Hello Professor Lavelle! I'm having trouble writing the formula for the following coordination complex: pentaaminebromocobalt (III). For the formula I got [CoBr3(NH3)5] but the correct answer is [CoBr(NH3)5]^2+. Could you please explain this problem? Thank you very much!
Hi, pentaaminebromocobalt (III) First, there are two ligands, amine and bromo. 1.pentaamine = (NH3)5 2.bromo=Br Secondly, the cation is cobalt (Co) and the oxidation number of cation is +3. Finally, check the oxidation of total complex. For amine, bromo, and cobalt, the oxidation is 0, 1, +3, respectively. The total oxidation is 5*0+(1)*1+(+3)=+2 Therefore, you can write down [CoBr(NH3)5]^2+ Best,
Professor Lavelle, on page 123 of the course reader, you state that "if R=8.314x10^2 L.bar.K^1.mol^1 is used then the answer is in bar which is perfect." I understand that the gas constant contains measurement of liters, which can cancel out, it contains temperature, which can cancel out, and it contains mol, which can cancel out;however, I remember that in your lecture you stated using the gas constant in relation to atm is the best way to go. Can you explain the difference between using bar and atm? Which is better? Which one should we use when answering a problem?
Hi, You can use either one. It depends on the question. But, make sure that the R is the correct one. For example, if you want to use atm and the R should be 0.082 L.atm.K^1.mol^1. If you use bar, then the R should be R=8.314x10^2 L.bar.K^1.mol^1. Just be careful the units. Best,
Dr. Lavelle, on page 118 in the course reader, 250 degrees Celsius is given for the problem. Throughout the process to solve the problem the temperature is not used. So is this temperature given to simply satisfy the fact that the three substances are gases? If not, what is the overall purpose of it? This is just to clarify whether the temperature will ever be used to solve for a concentration in problems such like these.
Hi, For the chemical reaction, we need to write down the conditions of this reaction system, including temperature, enthalpy, phase of substances, and so on. When do you need to use the temperature? It depends on the question. For example, if you want to convert the Kc to Kp, then you need to use the temperature. But for the problem on page 118 in the course reader, you don't need to consider the temperature. Best,
When do we use ferrate or iron when naming coordinate compounds?
Hi, If the complex ion is a cation, the metal is named same as the element. For example, Fe in a complex cation is called iron and Ni is called nickel. If the complex ion is an anion, the name of the metal ends with the suffix –ate. For example, Ni in a complex anion is called nickelate and Fe is called ferrate. Best,
Dear Professor Lavelle, I am having trouble with question 10.69: The reaction 2 HCL (g) reacts at equilibrium to form H2 (g) + Cl2 (g). K= 3.2 x 10^34 at 298 K. The initial partial pressures are H2, 1.0 bar; HCL, 2.0 bar; and Cl2, 3.0 bar. At equilibrium there is 1.0 mol H2 (g). What is the volume of the container? In the solutions manual the question is solved by changing the initial partial pressures to HCL, 4.0 bar; H2, 0 bar; and Cl2, 2.0 bar. Why would you change the initial pressures in this problem?
Hi, You can see that the Kc is extremely small (10^34). Therefore, you can image that this reaction is hard to move forward but easily push to the left side (reactant side). That's why you can set the new initial partial pressures and then calculate continuously. Best,
Hi Professor Lavelle! I have a question regarding the chapter 16 homework. Why is it that in number 27 B and D, cobalt is written as just cobalt but then in part C it is written as cobaltate? Does it have to do with the charge of the ligand being positive or negative? b. [Co(NH3)6]^3+ c. [Co(CN)5(OH2)]^2 d. [Co(SO4)(NH3)5]^+ Thank you!
Hi, After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element (eg. 27 B and D). If the complex ion is an anion, the name of the metal ends with the suffix –ate. For example, Co in a complex anion is called cobaltate (eg. 27C) and Pt is called platinate. Best,
In one problem the book gives the formula [Fe(ox)(Br)4]3 How come the (ox) comes before the Br, if alphabetically, Br comes before oxalato?
Hi, We consider the alphabetical order when we name the coordination compound. For this example, [Fe(ox)(Br)4]3 named as tetrabromidooxalatoferrate (III) ion. Best,
Hi Professor Lavelle, are the namings of the following compounds correct? These are from Ch 16.28: 1. [CrCl3(NH3)2(OH2)]+ as aquadiamminetrichloridochromium (IV) ion 2. [Rh(en)3)]3+ as trisethylenediaminerhodium (III) ion Thanks!
Hi, You need to consider the alphabetical order when you name the coordination compound. 1. [CrCl3(NH3)2(OH2)]+ as diammineaquatrichloridochromium (IV) ion. ammine should be in front of aqua. "m" is the higher alphabetical order than "q". 2. [Rh(en)3)]3+ as triethylenediaminerhodium (III) ion Best,
Hi professor. When calculating Kp, can we use partial pressures with any units? (atm, bar, Pa, etc.)
Hi, You can use partial pressures with any units. Then the units of Kp will be changed with units. ex. if you use atm and the unit of Kp should be related to atm. Best,
Hello Professor, I have a two part question... When writing the formula for coordination compounds, are there any rules regarding anionic and nuetral ligands? I have seen the coordination compound pentaamminesulfatocobalt (III) written as [Co(NH3)5(SO4)] and [Co(SO4)(NH3)5]. Do the rules change if there are atoms outside the coordination sphere? Ex. [Co(NH3)5(SO4)]Cl Thanks in advance!
Hi, Write down the names of coordination compound by the alphabetical order. Even it is coordination compound. Best,
Hi Professor Lavelle, In the textbook, it says "the number of digits following the decimal point in a pH value is equal to the number of sf in the corresponding molar concentration." So regarding sf in required pH value, it should still be the same as the least sf in calculation or it should follow the rule given by the textbook? Thanks!
Hi, Just follow the rule given by the textbook. Best,
Hello Professor Lavelle, I know changes to concentration of reactants or products, or changing pressure, etc do not affect the equilibrium constant, but does temperature? if so, how? and does adding a catalyst? if so, how? Thank You
Hi, The changes to temperature will also change the equilibrium constant. If the forward reaction is exothermic. Increasing the temperature decreases the value of the equilibrium constant. Where the forward reaction is endothermic, increasing the temperature increases the value of the equilibrium constant. The catalyst will not change the equilibrium constant. Best,
Hi Professor Lavelle, when writing the complex compound formula according to the given name, what does it mean by following the alphabetical order? It means the element, i.e. Cl should be put before NH3 because C goes before N or the naming, i.e. Cl should be put after NH3 because Cl is chloro and NH3 is aqua? Could you please specify for me? Thanks!
Hi, If there are two or more ligands in the coordination compound, you will need to name it by following the alphabetical order. For example, Cl and NH3 are two different ligands in the coordination complex. Cl named as chloro (chlorido) and NH3 named as ammine. According to the alphabetical order, a (ammine) should be infront of c (chloro). Therefore, you should write down ammine first. Best,
Hello, Professor Lavelle. I have a question regarding problem 16.27, letters (a) and (c). What is the difference between cyano and cyanido? It is said on the book that the ligand (CN)^1 should be called a cyanido. But on these two problems, the answers that are specified in the solutions manual are cyano (more specifically [Fe(CN)6]^4 > the answer in the solutions manual is hexacyanoferrate (II) ion while I got hexacyanIDO). Is there a distinction between the two? Thank you!
Hi, For a more systematic approach, the application of the ending ‘ido’ to all anionic ligands with ‘ide’ names in additive nomenclature. eg. cyanido and chlorido instead of chloro and cyano. Therefore, there's no difference between cyano and cyanido. You can pick either one. Best,
Hi Professor, for problem 16.31, can you explain how you know if a ligand can be polydentate? Thanks!
You can check its structure and if there is more than one atom with lone pairs, it can be a polydentate ligand.
Hello professor Lavelle, I have two questions, I hope that's okay. 1)When we are given a molecule, how can we determine if its a polydentate? Do we just memorize? For example en is bidentate and dien is tridentate. Should we just be aware of the molecules that are commonly polydentate? Can we draw out the lewis structure and determine if a molecule is a polydentate out somehow? 2)number 16.29c) asks us to write out the formula for tetraamminediaquacobalt(III) bromide; I came up with [Co(NH3)4(OH2)2] Br but the answer had a Br3 instead of a Br. I'm not sure why the Br3 is there. Can you please explain? Thank you!
1. A monodentate ligand has only one donor atom used to bond to the central metal atom or ion. Polydentate ligands have more than two donor atoms used to bond to a central metal atom or ion. It's better for you to memorize the common polydentate ligands and how they chelate to the central metal atom. 2. Because Co has three positive charges, and the ligands are all neutral here. So you need three Br to keep the compound neutral.
Hi Professor Lavelle. I am having trouble solving the following problem (#20) from the 2nd postmodule assessment, which states, "A researcher fills a 1.00 L reaction vessel with 1.84 x 10^4 mol of BrCl gas and heats it to 500 K. At equilibrium, only 18.3% of the BrCl gas remains. Calculate the equilibrium constant, assuming the following reaction is taking place: 2BrCl(g) ⇌ Br2 (g) + Cl2(g). How would I fill in the initial, change in, and equilibrium molar concentrations for Br2 and Cl2 in an ICE box (without knowing the initial concentrations of each molecule) if I take note of the fact that 2 mol BrCl <> 1 mol Br2 + 1 mol Cl2? I know that to calculate the equilibrium constant, I set up the equation as [Br2][Cl2]/[BrCl]^2, but I'm having trouble solving for [Br2] and [Cl2] at equilibrium. Thank you, Professor.
[Br2] and [Cl2] at equilibrium are determined by the reaction: 2BrCl(g) ⇌ Br2 (g) + Cl2(g). If we know how much BrCl is involved in the reaction (here, 100%18.3%), then we can calculate how much [Br2] and [Cl2] is produced. Given the initial concentration is set to 0, those are the concentrations at equilibrium.
Hey Professor, I have two questions. First, do we need to know how to convert from Kc to K, and vice versa. Also, for complex ions such as [FeCl3(h20)3]^, I was wondering why triaquatrichloroferrate(II) ion is the name and not triaquatrichloridoferrate (II) ion. Thanks.
You can use PV=nRT to convert it. Conversion process can be found in textbook. In this case, triaquatrichloroferrate indicates that this compound comes from substitution reaction. That's why there is a "trichloro" instead of "trichlorido".
Hi Professor Lavelle! I need help on Chapter 10 Question 67. "A 3.00 L reaction vessel is filled with 0.342 mol CO, 0.215 mol H2, and 0.125 mole CH3OH. Equilibrium is reached in the presence of a zinc oxidechromium (III) oxide catalyst and, at 300 degree C. a) As the reaction approaches equilibrium, will the concentration of CH3OH increase, decrease, or remain unchanged? b) What is the equilibrium composition of the mixture?" For part a), I understand that since Q > K, the reaction will proceed to produce more reactants. For part b), I am confused on what are the reactants and products. In the solution manual, the ice table has +x for CO, +2x for 2H2, and x for CH3OH. So, we are looking at CH3OH > CO + 2H2, right? This means that CH3OH is the reactant, and CO and 2H2 are the products. Yet, the solution manual, when calculating Kc, puts [CH3OH] over [CO][H2]^2. According to this equation, CH3OH becomes the product. Do you see my confusion? Thank you.
For Chapter 10 omit problems: 5, 43, and 67.
Hi professor. Can we use the "ice box" for pressure or do we need to convert it to concentration? And if yes what units of pressure can we use?
For gas, we can use ICE table for pressure. The standard pressure is given as 1 bar.
Hi professor. Can we use the "ice box" for pressure or do we need to convert it to concentration? And if yes what units of pressure can we use?
For gas, we can use ICE table for pressure. The standard pressure is given as 1 bar.
Hi professor Lavelle, After I read your solution for the question "If we make a reaction go faster, we can increase the amount of product at equilibrium" I'm confused. Since equilibrium constant is only influenced by the temperature. I don't think use catalyst would influence the amount of product at equilibrium, since catalyst basically only changes the rate of a reaction, not the amount of products. Could you please correct me if I'm wrong? Thanks!
You are right. If you make a reaction go faster, it means that you change the reaction rate. The amount of product will not be changed as long as the temperature stays the same.
Hi Professor Lavelle, I don't know how to convert the pressure of the gas into concentration without using pv=nrt, could you please tell me how to do so? Thanks
You should use pV=nRT.
For question 10.5a, how is the 3rd flask at equilibrium and the 4th not? Each of the two have 12 monoatomic and 6 diatomic molecules... Why can't it be flask 4?
Both of them are at equilibrium and flask 3 is the staring point. For Chapter 10 omit problems: 5, 43, and 67.
Hello, Professor, where can we see our quizzes and midterm scores online?
You can check that on MyUCLA.
Professor Lavelle, I was looking over a quiz problem in the purple book and I was left wondering why it was marked incorrect. The problem is written out like this: "Write out the electron configuration for N(base 2)+ and N(base 2)." I wrote out the electron configuration for N(base 2)+, for example, as [He]2s^2p^2. This was marked incorrect because it is not in molecular orbital theory configuration, using sigma and pie notation. So I'm left wondering why it was marked wrong when in the book similarlyworded problems appear and ask the answer to be in the same format that I wrote it in. The question is pretty much written to be answered in both ways, so it's unfairly misleading. I understand that the quiz is strictly on chapter 3 and therefore the question should be answered as was presented in the chapter, but what if a similar question appears on the final. How should the student determine what kind of configuration to write down? It would be very helpful if the question included something like "write out electron configuration for so and so in molecular orbital theory configuration."
[He]2s^2 2p^2 is the electron configuration for N+, not for N(sub 2)+.
Hi Professor Lavelle, For homework 10.5, question c, why can we use (12/17)*0.1? The question doesn't tell us the total pressure stays the same. It's true that the volume is the same since it's a fixed flask, but does that mean the pressure also stays the same? Thanks!
The pressure will increase. For Chapter 10 omit problems: 5, 43, and 67.
Hi professor Lavelle, For homework 10.43, we're given the partial pressure of NO and the concentration of N2. However, no temperature is provided, so how can I convert partial pressure to concentration? Thus, I don't think there is a right answer. If c is the right answer, could you please tell me why I don't need to convert? Thanks!
Yes, you are right. The question should read "x is the equilibrium partial pressure of N2(g)". Please check the latest announcement: For Chapter 10 omit problems: 5, 43, and 67.
Hi professor Lavelle, For homework 10.13, why should I put P before reactants and products£¿The question didn't say whether it's Qp or Qc Thanks!
For gas involved reactions, we always choose Qp.
Hi professor Lavelle, In one of the given solutions, you said " use partial pressure for gas and concentration for solution". For cases like a gas reacts with a solution. how can I decide k£¿I mean can I use partial pressure and concentration simultaneously without using pv=nRT to convert? If so, then what's the k would be? kp or kc? Could you please specify it for me? Thanks!
In this case, you should convert the pressure of the gas into concentration. Because the ideal gas equation pV=nRT only applies to gas.
Hi Professor Lavelle. For the true/false question that states, "If we make a reaction go faster, we can increase the amount of product at equilibrium," would the statement be false because the concentration of product does not change at equilibrium? Thank you.
If we increase the amount of product, the concentration of product surely changes. Why do you think it will not change?
Hi again, Could you please explain the following problem from the second equilibrium module? 16. A mixture of 2.5 moles H2O and 100 g of C are placed in a 50 L container and allowed to come to equilibrium subject to the following reaction: C(s) + H2O (g) â‡� CO (g) + H2 (g) . The equilibrium concentration of hydrogen is found to be [H2] = 0.040 M. Calculate the equilibrium constant Kc of this reaction. Thank you.
Just refered to the last question I answered. Use partial pressure for gas and concentration for solution, as for solid like C, just ignore it in the equation for K.
Hi Dr. Lavelle, I'm having trouble with the following problem from the first equilibrium module, could you please explain how to solve it? 30. You are studying the following unbalanced reaction at high temperatures: NH3 (g) â‡� N2 (g) + H2 (g) In one of your experiments at 500K the measured partial pressures are: NH3 0.887 bar H2 0.168 bar N2 0.0561 bar a) Determine the equilibrium constant for this reaction at 500 K. Thank you!
First, you need to balance the equation: 2NH3=N2+3H2. For gas, according to the definition of equilibrium constant: K=(PN2/P)*(PH2/P)^3/(PNH3/P)^2 where P is the standard pressure(100kPa, namely 1bar). In this way, K=3.38*104
Hi professor Lavelle, When we are asked to write the formula of a complex compound according to the given name, is there any order? Like alphabetical order or the number of same anions? And can I use abbreviation, e.g. use ox to indicate c2o4? Thanks.
In general, the chemical symbol of the central metal atom is written first, followed by the symbols for the ligands. Write the chemical symbols for the ligands in alphabetical order. If you are having trouble with a specific case, the "toolbox 16.1" section of your textbook on page 683 lays out both how to write the formula and name for coordination compounds.
Hi professor Lavelle, I'm a bit confused about the homework question 10.5 (a). I don't know what's the difference between the third and the fourth flask. Also , why the answer cannot be the second flask£¿ Thanks£¡
The flasks in 10.5a show the progression of the reaction X2 > 2X. In flask one, only X2 is present. However, in flask two, the reaction has begun to proceed and some X is present (8 X2 molecules and 6 X atoms to be precise). By flask three, we have 5 X2 molecules and 12 X atoms. The relative concentrations of X and X2 do not change from the third to the fourth flask, so we know that the reaction must be at equilibrium. Since flask three is the first flask with the equilibrium concentrations, it represents the point in time at which the reaction has reached equilibrium.
Hi professor. When calculating K with partial pressure, should we always use partial pressure values in bar units?
As long as al of the partial pressures you plug into the Kp expression are in the same units, it does not matter which units you use. Common pressure units include bar, Pa, and atm.
Hi Professor, if the reaction quotient is less than 1 (Q < 1) does that mean that since there are more reactants than products a forward reaction is favored? Or is that only true when Q < K, not 1?
Remember that when Q=K, the value is 1. Therefore, the products are favored when Q
Hi,Professor! In our textbook page384the graph showing the synthesis of ammonia, is the molar concentration of products always larger than that of reactants for all reactions at equilibrium? and why?
The molar concentration of products is not always larger than that of reactants at equilibrium. This will all depend on the specific reaction you are looking at. Imagine we have the following reaction: Br2(g) + Cl2(g) forming 2BrCl(g). If this reaction is at equilibrium, we can also write 2BrCl(g) forming Br2(g) + Cl2(g). As you can imagine, unless the equilibrium concentrations of reactants and products are equal, one of these reactions will have a larger molar concentration of products and the other will have a larger concentration of reactants.
Hi, Professor! How do you calculate the bond length of the SO bond in SO2? There are three kinds of resonance structures and two of them have one single bond and one double bond but another one has two double bonds. I don't think we can do that in the same way we do the nitrate ion.
The SO bond length in SO2 is between a single and a double bond since its resonance structures show resonance between single and double bonds. However, it would be difficult to estimate a numeric bond length since the proportion of single and double bond character in each SO bond relies on several resonance structures.
Hello,Professor! It is said in the course reader that under same conditions that the equilibrium constant does not change. So what do the conditions include? temperature? pressure? or anything else?
The only thing that changes an equilibrium constant is a change in temperature.
Hello Professor Lavelle, I have a question regarding dipole moments. When determining whether a molecule has a dipole moment, do we look at the overall molecule or individual bonds. For example SO2 looks like OSO (well, roughly looks like this). I know that a dipole moment happens when there is a polar bond and one atom is more electronegative then the other. I know that OS is a polar bond, and thus a dipole moment occurs. But do the dipole moments on the overall molecule cancel out and therefore there is no overall dipole moment?
Good question. You must not only look at the dipole moments of the bonds but also look at the VSEPR for that molecule. The VSEPR will tell you where the dipoles are going and thus you can figure out the dipole for the entire molecule.
Hi Professor Lavelle, is there any relation between an ion with high polarizing power and its coulombic attraction?
The polarizing power of a cation increases as the charge on the cation increases and as the size of the cation decreases. The coulombic attraction between two charges increases as the magnitude of the two charges increases and as the distance between the two decreases. In this way, we can see that there are some parallels between the two concepts. However, remember that polarizing power is referring specifically to one cation while coulombic attraction depends on two charged species.
Hi, Professor Lavelle. The solution manual indicates that the bond angles of TeF4 and (SbCl4) are larger than 90 degrees and larger than 180 degrees. Why not smaller? Since the lone pair push other atoms more tightly.
I believe the answer key for problem 3.71 has a typo. Both TeF4 and SbCl4 are seesaw shape. However, we know that the loan pair on the central atom will cause the bond angles to deviate from the ideal, giving smaller bond angles rather than larger bond angles as the answer key suggests.
Hi Professor Lavelle, For Midterm 2008 Q4A, what does it mean by I3 of Mg, I3 of Na? And have the smallest value of what? radius? energy? I cannot understand the question, could you please clarify it? Thanks.
In this question, the I is referring to ionization energy. Therefore, I3 of Mg means the third ionization energy of Mg. The first ionization energy is the energy to remove one electron from Mg, the second ionization energy is the energy to remove a second electron and so on.
Hello, for molecular orbitals I know we have to use one diagram for Z<8 and another for Z greater or equal to 8. but how do we determine the number of electrons. is it for the whole mocule or by the atoms involved for example N2 would have 10 valence electrons but individually would have 5 valence elctrons. what diagram should i use?
The number of electrons that go in the molecular orbital diagram is sum of the valence electrons contributed from each atom in the molecule. So, as you stated, the N2 M.O. diagram will have ten electrons since each N atom contributes 5 valence electrons.
Hi Professor Lavelle, When using constant numbers given with the test, do we need to automatically round off? Or we use what we're given? e.g. c=2.99792*10^8m/s. We keep it or we round up as 3.00*10^8 m/s? Thanks.
Use the constants that will be given to you on the exam.
Why don't the electrons involved in pibonds form hybrid orbitals?
Pibonds are only formed perpendicular to sigma bonds. Thus they are not involved in hybridization. Whenever you have double or triple bonds, you only look at piorbitals and not hybridization for those bonds.
Hello Professor, I am still not sure how to calculate the strengths and lengths of atoms. For example, on the quiz it asks us what the average bond length of CO bond length is 1.43A.o and the average C=O bond length is 1.12A.o. It asks us to calculate the carbonoxygen length in the carbonate ion. I understand that the triple bond is shorter and stronger than a single bond. But how can we calculate it?
You must first draw the correct lewos structure for carbonate ion. Then average out the number of double and single bonds given in the problem. There is no triple bond in carbonate ion.
Dr. Lavelle, for Fall 2011 Midterm, Question 2, the answer is given as 0.21 g, which is two significant figures. However, the question gives values such as 0.0100 moles, 50.0 mL, and 0.0500 M. Should there be three significant figures in this problem?
No your answer should be given in the smallest sig figs.
Hi Professor, if for the quantum numbers l=0,1,2...n1 then how come the highest is l=3? If there are 7 energy levels then wouldn't the highest be l=6? Thanks!
Yes that is true when we go up to G and H orbitals. However, this was not covered in this class.
Hello Professor, when drawing a molecular orbitals diagram, if Atom A's z<8 and Atom B's z>8, which diagram is used? Thank You. :)
For heteroatoms, use MO diagram for Z < 8 and draw A.O for the more electronegative atom slightly lower.
Dr.Lavelle, Is it okay to leave answers in the form of a fraction on the midterm or is this incorrect since significant figures are taken into account? Thank you
For the midterm, use decimals for your answers.
Hi for a quiz or midterm would it be considered wrong to point the dipole moment arowws in the direction of the positive charge like they do in the book and mentioned in the course reader? thanks
For the midterm or quizzes, point the arrow towards the more electronegative atom.
regarding the Question5.c in Fall 2008 midterm test, why can't the ml quantum number be 0, other than +1,or,1?
Hi Xuezhou, That question should be more clear. We mean the last electron as in the last one on the right side: _ _ _
Hello Professor Lavelle, a quick question about the midterm. The past exams have been 110 minutes and 110 points for 8 questions. Our midterm is 150 minutes and 8 questions, however, is it out of 110 or 150 points? Thanks!
Hi Sam, Our midterm is 110 minutes and 110 points with 8 questions. Just like the previous midterms.
If we are asked to write the ground state electron config, are we allowed to use noble gas config in the front and then write the rest out or are we expected to write out the full config. Would it be wrong if we decided to write the full electron config down (given we answered it correctly)?
Hi Michelle, Full electron configuration is never wrong :)
Hello, Looking at the past midterms I notice the point distribution for each problems. However, for certain problems, I used a different method for doing the problem and still came up with the same answer. I am answering the question properly but sometimes my work does not include some of the things you show in your solution because we did them in a different way. On the midterm, will the grading and point distribution depend on how each person does their problem and not necessarily by how the point distribution is shown in your answer key?
Hi Michelle, The partial credit is a guide. Some problems can be answered in different ways and partial credit is given (or full credit).
Hi Professor, I am doing a practice problem in one of the previous midterms where the question is: What is the maximum number of electrons in an atom that can have these quantum numbers? I am stuck on the one where it asks how many maximum number of electrons can be in n=5, ms = +1/2 The answer is 25, but I am not sure how to get to that solution. Could you explain this to me? Thank you!
n=5 indicates that you are looking at period 5 in the periodic table. ms indicates your spin quantum number. So you have to look in table 5 of the period table, and count the number of electrons with that spin number.
Hi professor. On the midterm, if we are off from the correct final answer by a tiny bit due to rounding differences, will we get points taken off?
As long as you round off at the end of your calculation, you should be fine.
Is the correct Lewis structure for O2 drawn with a double bond or a single bond as a radical?
The correct lewis structure for O2 in it's ground state is a double bond in between the two oxygens.
Hey professor, I was looking through the course reader and comparing the ways the valence electron configurations can be written by reading the MO diagrams. On the test, would we be allowed to simply write the valence electron configuration of O2 as (sigma2S)^2 (sigma2S*)^2 (sigma2P)^2 (pi2P)^4 (pi2P*)^2?
You have written the configuration correctly. However, do be careful on the midterm about writing the pi orbitals combined as you did here, since in some cases there will be unpaired electrons in those orbitals. For instance, (pi2P)^4 is fairly clear, but (pi2P)^2 is ambiguous and would be marked incorrect because we actually have (pi2Px)^1 (pi2Py)^1.
Hi Professor Lavelle, After I read answered questions listed below. I don't understand why bond angels of TeF4 are <90 and <120 while bond angles of TeCl4 is 90 and 120. They're both seesaws, why there would be difference? I don't understand why we have to take radius into consideration in this scenario. Thanks
In general, when we have the same central atom and same shape but different bonded atoms, the exact bond angles will vary. When the bonded atoms are larger, they need more space, repelling each other more. This gives rise to larger bond angles. When the bonded atoms are smaller, they do not take up as much space and the bond angles can be smaller.
Hi Professor! When making the lewis structures for molecules such as CBr2Cl2, H2CBr2, and AsF2Cl3, how do you know which elements are supposed to be next to each other or across from each other? For example, on the first one does it matter if the Br's are next to each other or across from each other in the lewis structure?
When drawing Lewis structures, you central atom will generally be the least electronegative atom (C in the case of CBr2Cl2 and As in the case of AsF2Cl3). To determine the exact arrangements of atoms around the central atom, think about properties such as atomic radius and electronegativity and how to make the most stable structure.
Hi Professor, I am reviewing the Course Reader notes, and I am confused on the difference between an element being a radical (explained on page 67 of CR) and a element being paramagnetic (explained on page 91 of CR)? Thank you!
If you draw the Lewis structure of the molecule and end up with an unpaired electron (think of the molecule NO), then that molecule is a radical molecule. Whether a molecule is paramagnetic or diamagnetic has to do with the molecules magnetic properties. When you draw the MO diagram of a molecule, such as O2, and end up with unpaired electrons in the molecular orbitals, that molecule is paramagnetic. However, if you draw the lewis structure of O2, you will clearly see that it is not a radical.
In Midterm 2011, Question 6B part C, it says LiH is the more ionic compound between LiH and HCl. Why is this true?
When comparing the ionic character of bonds, refer to the chart of electronegativities in your textbook. If you consult this chart, you will see that the difference in electronegativity values is greater in LiH than HCl, making LiH more ionic.
Dear Dr.Laurence, Do I need to know how to write the electron configurations for heteronuclear diatomic molecules for the midterms? There are several of them in the homework such as 79 and 61, and their schematic diagram is very different from that of diatomic molecules. Thanks
Anything asked in your homework is fair game for the quiz. When dealing with heteronuclear diatomic molecules, draw the M.O. diagram the same way you would for Z <= 8. In these cases, also make sure to draw the A.O.'s for the more electronegative atom slightly lower. You will be able to use your M.O. diagram to find the correct electron configuration.
On the homework problems 3.13B and 71B, the textbook asks for the bond angles of TeCl4 and TeF4 respectively. However, in the textbook answers, while they both are of the seesaw shape, TeCl4 is listed as having bond angles of 120 and 90 degrees, and TeF4 is listed as having bond angles of >90 and >120 degrees. Why is there a difference in the bond angles of the two compounds? And why are TeF4's bond angles are >90 and >120 degrees, shouldn't it be <90 and <120 degrees due to the fact that Fluorine has a smaller atomic radius than Chlorine?
This question was addressed in an earlier VOH submission which can be found below. Likely, the >90 and >120 are simply a typo in the book and should instead read as you suggested.
Hi Dr. Lavelle! I had a question on the 2009 Fall Midterm #6B. The question states "Are there one or two different bond lengths in ozone (03)?" The answer is "due to resonance, ozone only has one observed bond length". So whenever a molecule has resonance does that mean the bonds in resonance will be equal?
Yes, in general, when a molecule has resonance, the lengths of the bonds in resonance are equal. For instance, in ozone, the actual bond lengths will be in between the length of a single and double bond, and the lengths of each OO bond will be equal.
Hi Dr. Lavelle! I have a question regarding the Fall 2009 practice midterm. In number two you are asked to find the empirical formula of a compound given that the products of the combustion were 1.811 g of CO2 and 0.3172 g of H20. I found the g of C and g of H and understand that you subtract those from 1.000 g to get the grams of O, however, I do not understand why the grams of O is converted to moles of O... Why don't we just leave it in grams of O and then divide all three numbers by the smallest number out of those three? Thank you!
Comparing amounts of elements or molecules in grams is not the same as comparing amounts in moles. Imagine we are trying to determine an empirical formula and find that we have 1 g of C, 1 g of O, and 1 g of H. The ratio of these masses it 1:1:1. However, if we calculate the moles of each compound, we find that we have about 0.083 mol C, 0.063 mol O, and 1.0 mol H, giving a ratio that is clearly not one to one. When comparing amounts of elements or molecules (such as calculating empirical formula or finding limiting reactant), always convert grams to moles.
Hi Professor, could you please explain Coulombic energy or Coulombic attraction? We did not discuss it in class, and I feel the book doesn't do a very good job explaining what it is. Thanks!
A Coulomb is a unit of measurement for charge. Coulombic energy refers to the potential energy between two charged objects (repelling/attracting) and Coulombic attraction refers primarily to the attraction between positive and negative charges.
Hi, Professor! When I was doing problem 43 of chapter 3, I came across something like"scharacter" and "pcharacter", what do they mean and how do they affect the hybrid orbitals and bond angles?
s and pcharacter refer to the percentage that a hybrid orbital is made up of either s or p orbitals respectively. This means that the more s or pcharacter an orbital has, the more it is like that orbital (shapewise, energywise, ect.). For example: the sp hybrid orbital has 50% "scharacter" and 50% "pcharacter", the sp2 has 33.3% "scharacter" and 66.7% "pcharacter", and the sp3 has 25% "scharacter" and 75% "pcharacter". s orbitals have lower energy, and are shorter then p which have lower energy, and are shorter then d (assuming they belong to the same principle quantum state). Because of this, a hybrid orbital with more scharacter is lower energy and shorter then a hybrid orbital with more pcharacter and so on.
Hi Dr. Lavelle! If a question asks to draw a MO diagram for C2 (Fall 2010 quiz 2 prep #7), does it matter which atomic orbital gets the extra electron (the C on the left of the diagram or the C on the right side of the diagram)?
It makes no difference.
Hi Dr. Lavelle! In the Fall 2007 Midterm, question 3B states "What is the maximum number of electrons in an atom that can have these quantum numbers  n=5, ms = +1/2" The answer says 25 electrons. I would think the answer should be 16 electrons, because there are 16 total orbitals. Would you please explain why it is 25. Thanks.
The only way I can figure is that they are including a g orbital which can hold 18 electrons in its 9 suborbitals. These are currently no known elements that have g orbitals in their ground states but there would technecally be a 5g orbital which would lend 9 ms = +1/2 at most. Making the total: 1(5s) + 3(5p) + 5(5d) + 7(5f) + 9(5g) = 25 electrons which can be spin +1/2.
Hi, Professor! When I am working the problems in previous midterms I saw them using the number 3.00×〖10〗^8 as the speed of light instead of the one given on the front page. This will cause some differences between our answers and the standard ones? During our midterm, which one should we use?
Assuming the sig figs of the answer are not greater then 3, there should not be too much problem with using 3.00x10^8 m/s, however, you may end up with an answer that is off by a unit or two. We will not take off points for small errors (+/1 from the smallest sig fig rounding point) but it is generally always best to use the less rounded number in your calculations.
Hi Professor! I have a question about a problem in the book on page 125, number 19, part a. The solution manual draws the Lewis structure for CH2Cl2 where the hydrogen atoms are arranged on the left and top part of the carbon, while the chlorine atoms occupy the remaining spaces, making it a polar molecule. I was curious as to why the hydrogen atoms cannot be arranged so that it will be bonded to the right and left side, and the chlorine atoms to the top and bottom to make the molecule nonpolar?
The molecule CH2Cl2 has only 1 form in 3 dimensions. It's true structure is tetrahedral and therefore will be polar in any arrangement since dipoles of two atoms cannot be directly across from one another in this geometry.
Hello Dr. Lavelle. I'm looking at the wavelength range of ultraviolet radiation in the textbook, and it says the range is 400 nm to 100 nm. However, a couple of other sources said 400 nm to 10 nm. Can you clarify this detail? Thank you for your time.
I would say the 40010nm is a more accurate range. The naming of particular regions of the electromagnetic spectrum has to do with rough approximations for what types of energy transitions they induce.
Hello again, A few more questions: 1.)What is bond enthalpy? Is that something we will need to master before the exam? 2.) Number 71b in the Chapter 3 question section asks for the bond angle of TeF4. I got the right molecular shape (seesaw) but I would have thought that the angles would be: slightly smaller than 120 (since lonepairbonding pair repulsion is greater than bonding pairbonding pair repulsion), 90, and 180. However the answers given include >120 and >90. I don't understand the >120, or how there could possibly be an angle >90 (especially if they don't list a corresponding angle <90). 3.)I also don't really understand what scharacter is. Thank you for your help!
1) Bond enthalpy is essentially the heat energy released when a bond is broken. To my knowledge this is not on your test. 2) I would agree with you that the bond angles should be listed as 180, <120, and 90. Unless there is some factor I am unaware of, it is likely a typo. 3) scharacter refers to the percent that a hybrid orbital is made up by "s" orbitals. Such that sp has 50% "scharacter", sp2 has 33.3% "scharacter", sp3 has 25% "scharacter". The more "scharacter" a bond has the lower in energy it becomes, since s orbitals are lower in energy then p of the same principle quantum number (n).
Hi Dr. Lavelle, I have come across a few questions that involve long calculations (such as those regarding the photoelectric effect) where I try to enter all of my figures into the calculator at once (to avoid rounding between each step), and I think the calculator rounds for me, leaving me with, technically, the wrong answer. For example, it will read .00000025 and the answer is 2.54 x 10^7. Is there any way I can correct for this? Should I make a note on the exam that this was the most accurate figure I could get from the calculator? Thank you.
Your calculator is set on the wrong mode (Likely 'Flo' mode). Set it to either engineering or scientific to fix the problem.
Will we need to know how to do oxidationreduction reactions for the midterm?
All fundamentals and Chapter 13 in your book are fair game.
Hi Professor, I understand that HOMO stands for Highest Occupied Molecular Orbital and the LUMO stands for Lowest Unoccupied Molecular Orbital, but I'm not sure what those really mean. Thanks!
The name truly says it all. The highest occupied molecular orbital is the orbital of highest energy which holds electrons. For example B2 has the configuration: Ïƒ1s^2,Ïƒ1s*^2,Ï€2p^2 For this example Ï€2p would be the highest occupied molecular orbital (HOMO), as it is the highest energy level holding electrons. Now the LUMO is the next orbital in line energywise which holds no electrons (hence "unoccupied" by electrons). If I expand the B2 electron configuration to include its LUMO, it would read: Ïƒ1s^2,Ïƒ1s*^2,Ï€2p^2,Ïƒ2p^0 Ïƒ2p would be the LUMO for this example.
Hi, can you explain to me why the hybridization of the carbon atoms in benzyne (c6h4) are all sp3? The question is from the book chapter 3 number 93. thanks
I do not have the question in front of me, but by my knowledge, 4 of the carbon atoms in benzyne are sp2 and 2 are sp. This is perhaps a typo.
Hi Professor! I have a question from number 22 from the fourth module, which reads: Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 s1 before it can emit an electron from its surface. Answer the following two questions. B. If molybdenum is irradiated with 194 nm light, what is the maximum possible kinetic energy of the emitted electrons? For part B I am completely lost and don't know what to do. I saw that someone asked that earlier but I was still unsure of what to do after reading the response. Could you please help me with that? Thank you! Also, I am a little confused on kinetic energy vs. just regular energy because, for example, in number 20 part B asks for the energy of an incident light, but you are given the kinetic energy so I don't know what equation I would even use since I have the kinetic energy so what is the difference between the two? Sorry I am unsure of how to phrase that question. The exact question reads: In the second part of the experiment a shorter wavelength light source is used resulting in an ejected electron with a kinetic energy of 4.200 x 10^19 J. What is the energy of this incident light? What is the wavelength? I wasn't sure if I should ask my two questions separately or at once so I hope that's alright.
Before I answer your first question let me designate the three types of energy used in it (in answer to your second question): E(photon) = hÎ½, Energy carried by a packet of light (or photon only for light, not electrons). E(Ï†) = Energy required to remove an electron from a metal atoms HOMO to outside the atom. Ek = 1/2 mv^2, Kinetic energy or energy of motion (such as an ejected electron or a moving car NOT a photon). These energies can be transferred to the other energy types, particularly light energy transferring to eject an electron from a metal and then give it kinetic energy. The method for which these energies are calculated is by the equation: "E(photon) = E(Ï†)+ Ek" Now, the first question you asked gives you the frequency of a photon at the threshold energy of Mo metal, which can be plugged into the "E(photon) = E(Ï†)+ Ek" equation to get the threshold energy: We will call this photon, photon1. E(photon1) = E(Ï†)+ Ek = E(Ï†) + 0 = E(Ï†) = hÎ½ = (6.626E34 J*s)(1.09E15 1/s) = 7.22E19 J The second photon (which we will call photon2) has a 194nm wavelength. Using the same question we get: E(photon2) = E(Ï†)+ Ek = (7.22E19 J) + Ek = hÎ½ = hc/Î» = (6.626E34 J*s)(3.00E8 m/s)/(194E9 m) = 1.02E18 J Doing some algebra we attain: Ek = 1.02E18 J  7.22E19 J = 2.98E19 J which is the maximum kinetic energy of an ejected electron from 194nm wavelength light from molybdenum which has a threshold energy of 7.22E19 J.
Hi Professor. I just wanted to make sure that I am doing number 16 from the fourth video module correctly. The problem reads: Calculate the energy per photon of ultraviolet radiation of frequency 3.00 x 10^15 Hz. I used the equation E=hv/c and got an answer of 6.63 x 10^27 J. Is this the correct way to solve this?
The equation you used is a bit off: E=hÎ½, E = (6.626E34 Js)(3.00E15 1/s) = 1.99E18 J
Hi professor, I am having a little trouble with question 5C on the fall 2008 midterm. The question is as follows. "Give the first 3 quantum numbers for the last electron in a ground state bromine atom." I understand that n=4 and l= 1. However, when you start filling the p orbitals with the 5 electrons from bromine's valence shell, you notice that the last electron fills the middle orbital, which will have the magnetic quantum number (ml) 0 and not 1 or 1. Is this a typo? Thank you!
To my knowledge, the magnetic quantum number is somewhat arbitrary with regard to which gets filled first, second and third. Unless the suborbitals are specified like: 4p: _ _ _ (1) (0) (+1)
Hello, If a question asks to draw the molecular orbital diagram for a compound like N2(2) (nitrogen with a 2 charge), should one put the extra two electrons in the atomic orbitals portion of the diagram or simply allocate them to the molecular orbitals only? Also another question in identifying the HOMO/LUMO. If the LUMO consists of BOTH pi2px and pi2py should one say the LUMO is both pi2px and pi2py (since the energies are degenerate) or would saying pi2p be sufficient? Similarly if the HOMO consisted of one electron in pi2px and one electron in pi2py would one write HOMO = pi2px and pi2py or would simply saying pi2p be sufficient? Thank you very much!
Just to be safe put the additional electrons in both the atomic and molecular orbitals in the diagram. If I were grading it, saying just pi2p should suffice for both HOMO and LUMO (since this includes both in this case), but it is more accurate to specify the individual degenerate states.
Hello Professor Lavelle! When given the electron configuration of a particular element, how would we determine whether it is in the ground state or the excited state? For example, why is [Ar] 3d^8 4s^2 considered to be in the excited energy state and [Kr] 4d^1 in the ground state? Thank you very much! Jasmine Heyer
The ground state is when the electrons in the atom (or molecule) are configured in the lowest energy state with regards to orbotals energy levels. The excited state is when electrons (usually from the outmost orbital) are excited to a higher energy level. For example: Ground state Li: [He]2s^1 One example of excited state Li: [He]2p^1 I am unsure which elements/ions the second part of your question reffers to or I could be more explicit.
Hello, Professor. If the measured value is an integer, how many sf should we keep for the answer value? how many sf should we keep for the Molar mass?
For the best accuracy, take out the molar mass as many sig figs as you can. Once you have the answer, round it to the correct number of sig figs according to the sig fig rules.
Hello,Professor. Can we decide whether a molecule is polar or nonpolar by its Lewis structure? Can we draw the dipole moments on the Lewis structure?
You can certainly draw dipole moments on a lewis structure (And should if asked). As far as polarity goes if there are lone pairs on the central atom, the molecule will be polar (except in rare cases like square planar). If the central atom has no lone pairs, then polarity depends on symmetry. If the other elements are symmetric around the central atom such that their dipoles cancel, it will be nonpolar. This is difficult to show with on lewis structures explicitely (except with 2d structures like trigonal planar and linear) without drawing out the 3 dimentional structure.
Hi Professor, I was doing some of the practice problems in the textbook and it says that the hybridization for a Seesaw shape is dsp^3. Shouldn't it be sp^3d? Does it make a difference? Thank you
Hi Lawrence, As discussed in class dsp^3 or sp^3d are both fine. Some books use dsp^3. Remember all 5 hybrid orbitals are degenerate (same energy).
Where can I watch the review session video? I checked bruincast website but the video was not uploaded ):
Hi Hojin, See Announcements on class web site. Also Reminder to see class link: Important Midterm Information
Hi Professor. I was trying to do number 15 from the third module, reading: If an 800 mL aqueous solution contains 12.4 g of ethanol, CH3CH2OH, what is molarity of ethanol in this solution? I found the moles of ethanol to be .1849 moles and then divided that by .8 L, which gave me .231125, but that is not an answer choice. I am not sure what I am doing wrong Thank you!
The molar mass of Ethanol is 46.07g/mol (12.4g)(1mol Ethanol / 46.07g) = 0.26916 mol Ethanol 0.26916 mol Ethanol / 0.8L = 0.3364 M Ethanol (Or 0.3 M if that 800mL was truly given as 1 sig fig). Try recalculating.
Hi Dr. Lavelle, Regarding Number 6 on the Fall 2010 Midterm: Why is it that when describing the composition of sigma bonds, we list the full C2sp2 or O2sp2, whereas for the pi bonds we only list C2p and O2p? Similarly, what is the reasoning behind defining the lone pairs on oxygen as Osp2 (as opposed to O2sp2, or O2p)? Thank you!
Because pi bonds are composed of p (and sometimes d for metals) orbitals, not hybrid orbitals like sigma bonds. Also, lone pairs are held in hybrid orbitals as well in order to maintain the best structural geometry such that the regions of electron density are as far apart as possible. Now I do not have the question in front of me, however, I see no reason why the lone pairs on oxygen can't be said to be in O2sp2 (instead of Osp2).
Hi professor! I was wondering if Friday's lesson will be included on the midterm Wednesday. Thank you!
The midterm will cover chapters 1, 2 and 3 of your book in their entirety. Any remaining chapter 3 topics covered in Friday's lecture will be included as well.
HI! I was trying to do number 10 from the empirical and molecular assessment video: LDopa, a drug used for the treatment of Parkinson’s disease, is 54.82% C, 5.62% H, 7.10% N, and 32.46% O, and has a molar mass of 197.19 g/mol. I was able to find the empirical formula and I know that to get the molecular formula I have to find the molar mass of C9H11NO4 (which is what the empirical formula came out to be), and then divide it by another mass, but I am not sure where I get the other mass from so that I can multiply the empirical formula by that to get the molecular formula. Sorry that was really difficult to type out. But hopefully it made sense.
As it turns out, C9H11NO4 is both the empirical and molecular formula for this question. The way you know is by dividing the given molar mass by the empirical molar mass and then multiplying all coefficients by the resulting value. In this example the given mass was 197.19g/mol which would be divided by the molar mass of C9H11NO4 which is also 197.19g/mol. The result is (197.19/197.19)=1, so therefore all coefficient are multiplied by 1: C9(x1)H11(x1)N1(x1)O4(x1) = C9H11NO4 the molecular formula (which again, is the same as the empirical for this example).
Hi Professor Lavelle! For sodium bisoxalato(diaqua)ferrate(III), is it okay to write it as Na[Fe(C2O4)2(OH2)2] instead of Na[Fe(OH2)2(C2O4)2], considering that ligands are written in alphabetical order?
The correct naming order should be alphabetical as such: sodium diaquabisoxalatoferrate(III) which corresponds to Na[Fe(OH2)2(C2O4)2].
Hi Professor! Question 9 of Quiz 2 prep Fall 2010 involves the molecular orbital of heteronuclear diatomic molecule, which is not covered in class but appears in the textbook. Are we supposed to know how to draw them or not? Thanks!
Certainly if the heteronuclear diatomic molecule involves two atoms which are both z>/=8 or z<8. For situations where the heteronuclear diatomic molecule involves one atom from z>/=8 and one from z<8 it get a little more complicated and you will likely be given enough information to know which order the pi and sigma 2p orbitals come in.
Hello professor, When we have a Carbon that has an electron in one of the 2sp3 orbitals that bonds with a Hydrogen atom in a 1s orbital, what happens if the spin states for both the Carbon atom and the hydrogen atom are in the spin up state? Would there be a form of repulsion or would the Shrodinger equation for one of the atoms have to be modified?
This is not an issue you will have to deal with in Chem14A, but typically if two atoms collide and their outer electrons are in the same spin states one of the electrons will change states (assuming they have enough energy to do so).
Hello Professor!!! On Wednesday, we discussed the diagram that correlates with molecular orbital theory and how it varies when z>8 and when z<8 with homonuclear diatomic molecules. However, when the molecule is heteronuclear, for instance NO, where do you put the sigma 2pz orbital? Is it above the 2pi bonds, like when z > 8, or below them, similar to when z < 8? Thank you so much!!!
For heteronuclear diatomic molecules where one is part of the z>/=8 and one is in the z<8 there is no sure fire remedy. For the example you gave, NO has the two pi 2p bonds are lower then the sigma 2p.
Hi Dr. Lavelle, I have a few questions: In our notes on hybridization, the first example is the orbital energy level diagram for the carbon atom in CH4. In light of what is said on the next page about the conservation of orbitals, I do not understand why the diagram for regular carbon has three (two electrons in 2s, and one each in 2p x and y) while the hybridization has four. I believe the sp3 represents one s, and three p orbitals, which makes the four so that part makes sense. It's the conservation of orbitals that seems to be missing, from what I understand. In the next example, the nitrogen in NH3 also has sp3 hybridization, but it at least has four occupied orbitals to begin with (2 electrons in 2s and one each in 2p x, y, and z). Can you please explain this difference? The rest of my question are regarding the Fall 2008 Midterm. Number 4 asks "Which of the following would have the smallest value?" and lists "I3 of Mg, I3 of Be, I2 of Na, I2 of Mg, and I2 of Li." I have absolutely no idea what this question is asking or what the answers mean. Could you please explain? Number 5C asks for the first three quantum numbers for the last electron in a groundstate bromine atom, and gives 1 or +1 for ml. How can it be either one? Thank you for your help.
Your first question is hard to follow, can you please try making a more specific question. When you have four bonds around an atom like in CH4, you are using the s orbital and all three p orbitals. Accordingly in NH3, when you have three bonds around your central atom and a lone pair, you are also making use of all 4 orbitals, the s and 3 p orbitals.... For the second part of your question, Fall 2008 Midterm Q4A is referring to Ionization energies of the given elements. The question is asking, "Which of the following would have the smallest value (of ionization energy)?" because "I" refers to ionization energy. I2 refers to the second ionization energy (the energy required to remove a second electron from that atom), and I3 refers to the third ionization energy (the energy required to remove a third electron from that atom). In general, the trend of ionization energy increases as you go across the periodic table and decreases as you go down the periodic table. In terms of decreasing first ionization energies, the trend would be: Be, Li, Mg, Na. After the removal of the first electron, Be would have an electron configuration like Li. Li would have an electron configuration like He. Mg would have an electron configuration like Na. And Na would have an electron configuration like Ne. For the second ionization energies, the electron configurations with now noble gas electron configurations will be very reluctant to give up those configurations and in turn will take a lot of energy for a removal of a second electron (in this case, Na and Li). On the other hand, Mg and Be now have almost noblegas configurations and their second ionization energies will be low because they are happy to give up that last electron and achieve noblegas electron configurations. Furthermore, Mg's second ionization energy is lower than Be's because of the initial trend that ionization energy decreases as you go down the periodic table. The last two scenarios that we have not yet mentioned, the third ionization energies of Mg and Be, will be very large because at this point, they have also achieved noblegas electron configuration and are reluctant to give it up. In conclusion, that is why the second ionization energy of Mg has the smallest value. For your third question, ml refers to the specific p orbital that the electron may be in. By convention, we write the values of ml for the p orbitals as 1, 0, +1. As you fill the p orbitals, the last electron you place in the p orbitals should technically go into the 0 orbital based on this convention. Any value of ml for the p orbitals (1, 0, +1) for this situation should be accepted.
Hi Professor! A question in the textbooks asks us to explain the high reactivity (instability) of fluorine gas using the MO theory. As is noted in the course reader that this instability results from the repulsion between the 3 lone pair electrons. So, are these pairs of electrons the ones in the destabilizing orbitals (sigma*2s, pi*2px and pi*2py) in the MO diagrams? Thanks!
From a Lewis dot structure view, there are 3 lone pairs on each fluorine. From a molecular orbital theory view, there are two bonding electrons and two antibonding electrons in the 2s orbitals (sigma 2s)2(sigma 2s*)2. In the 2p orbitals, there are 6 bonding electrons and 4 antibonding electrons (continuing on with the electron configuration we have (sigma 2pz)2(pi 2px)2(pi 2py)2(pi 2px*)2(pi 2py*)2) Because the HOMO (highest occupied molecular orbital) is an antibonding orbital and the overall bond order is 1 (relatively low), the molecule is relatively unstable and reactive.
Hi Dr. Lavelle, I have two questions: 1) Why does the least electronegative atom always go in the middle of a lewis structure? 2) In class we only covered fairly simple lewis structures. How do we know which atom attaches to which in more complicated molecules like the ones given for the homework? (XeOF4, HXeO4^, OC(NH2)2 etc.)
There are many reasons the least electronegative atom is generally the central atom in Lewis structures. There are also many ways to think about it. I like to think about it this way, the least electronegative atom is the central atom in covalent structures because it is the most willing to share its electrons. The second part of your question has two answers. For the first two examples you gave, and in general for inorganic compounds, the least electronegative atom being the central atom will nearly always work. When in doubt, count the valence electrons, pick the central atom, place the other atoms around the central atom, and place your remaining electrons around the atoms, trying to follow the octet rule and keeping in mind formal charges. For the last example you gave, and in general I believe you are referring to organic compounds, the Lewis structure can vary on the types of organic functional groups present in the compound. In this case, since you are not yet familiar with organic functional groups, we will try to give you hints about the skeletal structure of the compound. In your example, having (NH2) in brackets means that there is a nitrogen atom with two hydrogen atoms attached to it. When in doubt or without hints, place your carbons in a chain, (unless it is a C6H6 type molecule and then you know you are talking about a benzene ring) place your remaining atoms around the carbons, and try to make something that seems logical with your valence electrons, octet rule, and formal charges.
Hello Professor, on pg 82 of our textbook, the covalent radii for H and O are given at 37 pm and 66pm (for a single bond), respectively. How come, then, in the text on that page they calculate the OH bond in ethanol as 37pm + 74 pm? Why don't they use 66 pm for the O bond, as given in the bottom table? Thanks!
I believe this is a typo. You are correct, for a single bond between oxygen and hydrogen, where there is no resonance to be considered for this bond, the bond length should be (37 pm)+(66 pm)= 103 pm.
Hello Professor Lavelle! For the midterm, is it possible that we will be asked to state the electron configuration for any diatomic molecule? Or only homonuclear diatomic molecules? Thank you very much!
You will only be asked to state the electron configuration for homonuclear diatomic molecules as those are the only molecules you will be required to draw the molecular orbitals for.
Hi professor, for fall 2011 midterm Q6A, why is the electron configuration for Cu+: [Ar]3d10, but not [Ar] 3d84s2? Thank you!
Copper, in its elemental form, has one of the exception electron configurations where it takes one of the electrons in the 4s orbital and puts it in its 3d orbitals so that the d orbitals can be full (d orbitals are generally more stable when they are halffull or totally full). In this case, copper has an electron configuration of 1s22s22p63s23p63d104s1, or in shorthand, [Ar]3d104s1. To make copper a ion with a (+) charge, the electron in the 4s orbital would be removed, and the electron configuration would then be 1s22s22p63s23p63d10, or shorthand [Ar]3d10.
In the midterm fall 2011, Question 4B, should the answer 452nm instead of 450nm because there are three significant figures. Thanks
As written in the answer key, the answer is 450. nm, which is 3 significant figures.
Hi Professor, if the bond order is not a whole number, 2.5 for example, does it mean that this molecule has two bonds plus one unpaired electron?
No. Unfortunately, molecular orbital theory/bond order and Lewis dot structures are not always directly correlated. Remember, Lewis dot structures are like a snapshot of what the most stable electron arrangement would be around the atoms, it is not a full conceptual picture of how the electrons are behaving.
Hi Professor, For a molecule such as NO3, there are 3 different resonance structures, with 2 types of bonds that are blended amongst 3 oxygen atoms correct? When calculating bond length, would you only calculate the length of the double bond and the single bond and then average the two (i.e. [136 + 115]/2 = 125.5), or would you calculate those values and incorporate the other single bond? (i.e. 136 + 115 + 115, because there are 3 oxygen atoms, and then divide by 3).
There are 3 resonance structures for the nitrate ion, where the double bond between nitrogen and oxygen moves to the other oxygen and nitrogen bonds. At any given time, there is one double bond between an oxygen and nitrogen, and two single bonds between the nitrogen and the other two oxygens. To calculate the bond length between the nitrogen and oxygen bonds, it would be (2/3) single bond length (141 pm; 75 pm for N and 66 pm for O according to Pg. 82 in the textbook) and (1/3) double bond length (120 pm; 60 pm for N and 60 pm for O). The calculation for the nitrogenoxygen bond length in the nitrate ion: [(2/3)*(141 pm)]+[(1/3)*(120 pm)]= 134 pm
Hi Professor Lavelle, For bonding in heteronuclear Diatomic Molecules, we should still use ¦Ò2s, ¦Ò2s* to represent orbitals, or we can simply use 1¦Ò, 2¦Ò to represent orbitals as the textbook mentioned? Also, do we need to know orbitals in polyatomic molecules? Thanks!
Please rewrite the question so that we can see the correct symbols. For this course, you only need to be able to write molecular orbitals for diatomic molecules.
Hey Professor! I was just wondering how to calculate the bond length. I know many times a chart or the actual lengths are given, but do we simply add the numbers? Or do we have to divide them by the number of atoms the central molecule is bonded to?
For each bond, you add the numbers given for that type of bond (i.e. single bond, double bond, etc).
Hi Professor Lavelle. Do we need to know how to write the electron configuration of heteronuclear diatomic molecules or polyatomic molecules? Thanks.
You might be asked to know the electron configuration for heteronuclear diatomic molecules, however, generally you will be given two with Z<8 or two with Z
Hello Professor, notation wise for Molecular Orbital Theory when stating the electron configuration, is it better to state σ2p or σ2pz for the subscript? Thank you!
Use the more specific notation (σ2pz), as that is what your book uses.
Hi Professor Lavelle. I have a minor sig fig question. Does the number of sig figs of the solution's preceding calculations take precedent over the number of sig figs given by the data in the original question? For example, in the solution to Question #2 on the Fall 2011 Midterm in the course reader (the question deals with calculating the mass of an excess reactant), the answer has 2 sig figs (whereas the data in the question has values all with 3 sig figs). I understand that in the process of calculating the solution, the least amount of decimal places were taken into account (in the step where 0.0100 mol  0.00750 = 0.0025). But does the sig fig number concluded in the calculations outweigh the sig fig number given by the question's original data? Thank you, Professor.
When multiplying number to reach an answer, use the lowest sig fig number as the sig fig number of the answer (2.03*1.0 = 2.0). When adding or subtracting, you use the smallest decimal place for the answer (2.005 + 5.6 = 7.6). When required to do both in sequence, the answer is restricted in both ways (1.00*10.0 + 1.00*10. = 10.0 + 10. = 20.).
Hi, Professor! When we consider the most important structure of resonance, like N3^1, which one is more stable? the one in which the FC of three N respectfully are1,+1，1 or the one the FC are 0,+1,2?
Leaving a larger charge on a single atom is generally less stable then spreading out the charge. Therefore the 1,+1,1 state is preferred.
Hi Professor Lavelle. I was wondering how you might use the molecular orbital theory to determine the bond length of a molecule? ie if you wanted to list molecules by length
There are no calculations specifically for bond length needed for this class, however, you can calculate bond order using MO theory (BO = 0.5[# bonding electrons  # antibonding electrons]) and use it to determine relative bond lengths (the higher the bond order, the shorter the bond length).
Hi Professor Lavelle! In problem 13 of the Chapter 3 homework problems, it asks us to predict the approximate bond angles for TeCl4 (part c). The resulting VSEPR shape for this molecule is the see saw shape. How come the equatorial angle of ClTeCl is 120 (answer from the book) and not less than 120? Wouldn't the angle be less than 120 because of the lone pair? Thank you.
In reality all of the bond angles are distorted in various molecules due to each specific electronic environment. The seesaw geometry is a rather unusual one with above average variation in bond angles (since it has 3 distinct ones). As a result of this complication it is easier to just leave its angles as 180, 120, and 90 which are a somewhat average of various seesaw bond angles.
Hi! On the homework for chapter 2, a lot of the questions ask about drawing lewis structures and sometimes it will specify when it wants the resonance form of the structure, but it doesn't specify which lewis structure to draw (in terms of formal charge) so there ends up being many answers. On our quizzes and tests, will it be specifically stated that a lewis structure I am drawing should be in the resonance form, or can it be ANY correct lewis structure? Am I trying to draw the most probable lewis structure (atoms with formal charges closest to 0) ?
If it has a resonance form, the safest bet is to write all of them out if the question is not specific (reasonable resonance forms will involve observing the octet rule for N, C, O and F). If the question asks for the most stable, however, you need only draw the most stable (usually with the lowest amount formal charge).
Dear Professor Lavelle, When asked to write the Lewis Structure of a molecule, if it's a resonance structure. We should write all or we only need to write the one with lowest formal charge? Thanks.
If a question specifies for you to write the most stable resonance structure, then you need only draw one structure (where the most stable structure usually has the lowest formal charge). If instead you are asked to show all resonance structures, then you must draw all reasonable resonance structures.
For question 57a, chapter 2, why does the lewis structure for chlorine monoxide assign 6 valence electrons to CL and 7 valence electrons to oxygen?
The lewis structure for chlorine monoxide consists of a single bond between the two atoms, 3 lone pairs on clorine, 2 lone pairs on oxygen and an unpaired electron on oxygen.
Dear Prof. Lavelle, For O2, would it be better to draw the Lewis structure with a double bond and 2 lone pairs per O atom, or with a single bond and 2 lone pairs and one unpaired electron per O atom? Because according to the Fall 2010 quiz, questions #7 and 8, the answers seem to be derived from the latter case.
Now that you are aware of the true bond structure of O2 (thanks to molecular orbital theory), you will be expected to draw O2 as a diradical.
Hi Professor Lavelle. In Chapter 1 homework questions, for like Ru3+, the book writes the electron configuration as [Kr]4d5. I don't understand why it wouldn't be [Kr]5s24d3 because doesn't the 5s orbital fill before the 4d orbital? Thank you.
For transition metal ions and posttransition metal ions (before the metaloids) d orbitals are lower energy then s or p and therefore fill first.
Hello Professor, On one of the homework problems, the electron configuration for Ga3+ was not the same as the normal electron configuration for Ni. The periodic table lists the Ni configuration ending in 4s2,3d8 whereas the Ga3+ ion's configuration ended as 3d10. Is there a reason why the 2 electrons filled in the d shell instead of the s shell in Ga3+, and in Pd, but not in Ni? Do all ions of elements in groups located right of the d block act this way by filling in the d orbital before the s?
Typically all main group metal ions including those before the metaloids (which includes gallium) have lower energy d orbitals then s and p. As a result the s and p orbitals are removed first leaving only d orbitals remaining. Because of this Ga3+ is [Ar]3d10, In3+ is [Kr]4d10, Ni2+ is [Ar]3d8, and so on. Isoelectronic elements often do not share identical electron configurations due to changes in effective nuclear charge and subtle differences in orbital energy levels (particularly for 4s and 3d).
Professor: When answering the first few homework questions for Chapter 3, sometimes the bond angle for AX3E2 is said to be less than 109.5 degrees (11c 17d), and sometimes it is said to be 109.5 degrees (13d). Is this a mistake in the book, or are there some molecules where the angle equals 109.5 degrees and some in which the angle is less? If so, how can we tell the two kinds of AX3E2 molecules apart?
AX3E2 has the geometry of Tshape which has bond angles of 90 degrees and 180 degrees.
I'm having trouble understanding why the shape of a ClO2^1 ion turns out to be angular rather than linear. I would assume the two lone pairs on the chlorine would equally repel each other, thus one would not repel another to form an angle.
Just as in the case of water (H2O), ClO2^1 has two lone pairs and two atoms attached the the center atom. All four of these are regions of electron density which want to form the most stable structure for that configuration. We know that for four regions of electron density the structure is tetrahedral, not square planar. Therefore the geometry for AB2E2 must be bent seeing as two of the positions are lone pairs and not atoms. The same can be said about ammonia. It has a tetrahedral structure when taking it's lone pair into account, but because it has only three atoms attached, its geometry is trigonal pyramidal and not trigonal planar.
Hi Professor Lavelle, In the course reader, it's stated that the bond angles of an octahedron are 90 degrees but in the solution manual and study guide, it says that the angles are both 90 and 180 degrees, which one is correct? Thank you!
There are both 90 and 180 degree angles in an octahedron.
Hi Professor, Why is the shape of H20 bent but not linear? Thank you!
Because the oxygen (which is the central atom) has lone pairs which take up space in the same way as bonds (but without an attached atom).
Hi, professor, why is O3(ozone) polar? Thank you
Ozone is polar because the three oxygen atoms are not in the same electronic environment. The center oxygen has electron density pulled by the two adjacent oxygen atoms unsymmetrical causing a net dipole (because of the bent structure due to the lone pair on the center oxygen). As for the fact that all three atoms are oxygen, consider the following analogy: Imagine two people of equal strength pulling on a rope against each other. They would be deadlocked in position. Now imagine three people of equal strength, one in the middle holding two ropes and the other two each pulling on one of the ropes in nonopposing directions. The middle person will be pulled in which ever direction the two side fellows agree upon (as in not opposing each other) since there combined strength is greater then the middle persons, even though they all have the same strength. The same is true for electronegativity (or electron pulling power in a bond).
For number 1 on the practice fall 2010 question, will points be deducted if we answer with d2sp3 instead of sp3d2, or are the two answers the same?
Yes, the order of the orbitals is important seeing as they are in order of increasing energy.
Hi Professor! Question 19a in chapter three asks to draw the Lewis Structure of CH2CL2. How do we know that the chlorine atoms are arranged cis and no trans? Thanks!
Think about the tetrahedral geometry of the CH2Cl2 molecule. In such a geometry there are no cis or trans configurations.
Hi Professor Lavelle. When asked to write the groundstate (or full) electron configuration of an element on a quiz or exam, would it be more appropriate to start with the nearest noble gas (i.e. [Ar]) or start our configuration from 1s? Thank you.
Start the configuration from 1s¡£
Dear Professor Lavelle, Do we need to know the properties of compounds for midterm? For example, phosphorous trichloride is a toxic, colorless liquid? There are a lot of descriptions about properties on the textbook.
No need to remember that.
Hi! I was trying to do number 2.81 from the homework, but I don't think the book does a very good job of explaining the trends for anion polarizability. I understood the one for cations pretty well but it did not explain much for anions. Could you please explain how that works? Thank you!
The radii orde of the 4 anions is O2
Hi Professor Lavelle. For homework problem #11 (Part A) in Chapter 3 of the textbook, the question asks you to determine the shape and bond angle for I3. The molecule's Lewis structure has three I atoms arranged in a linear shape with 3 lone pairs of electrons surrounding the central I atom. I don't understand why the two lone pairs of electrons on one side of the central atom wouldn't push the surrounding I atoms away from them to create a bent shape. Would you be able to explain why the molecule's shape is linear and not bent? Thank you!
The arrangment of three lone paires is not just putting one lone pair in one side of central I atom, and other two putting in opposite side. The three lones pairs have certain angle with each other.
Hello Professor, I was wondering, would we have to know or memorize bond strengths between certain atoms? Do we need to know specific dissociation energies also? We didn't go over these sections in class in detail, but I understand that the information is readily available in the textbook. Thank you for your time!
You don't need to memorize specific dissociation energies and bond strengths.
Hey professor! When we have to write the entire electron configuration of an element that contains d orbitals for instance Gallium... do we write the d orbitals first and then the s or do we write the s orbitals first since they get filled up first and then the d? So for Gallium would the electron configuration be 1s22s22p63s23p63d104s24p1 or would it be 1s22s22p63s23p64s23d104p1 ?
1s22s22p63s23p63d104s24p1 or 1s22s22p63s23p64s23d104p1 both OK
Hi Professor Lavelle! Can you explain this statement: "As the scharacter of a hybrid orbital increases, the bond angle increases"? Thank you.
Because as the scharacter of a hybrid orbital increases, it will be more conducive to atomic orbital being maximum overlap.The bonding ability of hybrid orbital is stronger than the original orbital. So the bond angle increases
Hi Professor Lavelle! For P4, how do you tell whether it is polar or nonpolar?
P4 is nonpolar, its tetrahedron shape is symmetrical structure.
Hi professor. Does greater polarizing power and polarizability cause greater covalent character or ionic character?
Greater polarizing power and polarizability cause more covalent character.
Hi Professor, when doing the electron configuration for Cl+ I know it's [Ne] 3s23p4, but wouldn't it be more stable if it were [Ne] 3p6? Also, does resonance correlate with polarity in any way? e.g. O3 is polar and has 2 resonant structures. Thanks
The [Ne] 3s23p4 is more stable. Polarity correlate with electronegativity. Resonance makes the bonds stronger.
Hi Professor Lavelle, If the material being covered on the quiz only goes up to 3.3 in the textbook, then how much of the practice quizzes are we going to be expected to be capable of completing? It would seem that those quizzes in the purple work book cover chapter 2 and all of chapter 3, beyond 3.3. To put it more concisely, how much of the material must we complete in order to receive the extra credit for quiz 2? Thank you!
You should turn in your workbook next week.So next week you can complete the preparation for quiz 2.
Professor Lavelle I'm a bit confused on ionization energy versus electron affinity versus electronegativity. Can you explain the similarites and differences between the three? Also when we write a Lewis Structure, the element that is placed in the middle is the one with the lower ionization energy correct?
Ionization energy £ºan atom or molecule, is the energy required to remove electrons from gaseous atoms or ions. electron affinity is the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion. Electronegativity is a chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself In Lewis Structure the element that is placed in the middle is the one with the lower electronegativity.
Hi professor. In the molecule CH2Cl2, do we need to arrange the surrounding atoms in a specific way? For example, should we put the Cl atoms opposite of each other or next to each other? And are all these arrangements polar?
In the molecule CH2Cl2, you need to arrange the surrounding atoms in a specific way. because the two Cl atoms is not in opposite position.
Hello Professor Lavelle, a question about ions in the periodic table. Are we supposed to memorize all the ions of the elements? For example, Osmium^4+, Dysprosmium^3+,Tantalum^5+? Or just certain ones?(could you name the ones we need to know) Thank you!
No need to memorize all the ions of elements.
Hi Professor, I am having trouble with number 83 in Chapter 2, which asks us to place a set of molecules in order of decreasing bond length. Part b asks for the length of the SO bond in SO2, SO3, and SO3(2). I would have thought that both forms of SO3 would be tied for shortest length, because they can both be written with all double bonds (the S in SO3(2) would have an additional lone pair), while SO2 must have at least one single bond. The answer, however, gave SO3(2)> SO2=SO3. Could you please explain? Thank you.
Because in SO3(2), there are three Lewis structure that have a zero formal charge at S. Each has one SO double bond and two SO single bond. These SO bond would have a substantial amount of single bond character, so they would be expected to be longer than those in SO2 and SO3
Hi, professor! What does MO mean in the course reading P69? Lewis structures with expanded octet have lower energy than octet structure( compare FC&MO)
Molecular Orbital.
Hi professor Lavelle. If we have the choice between giving a 1 and 0 formal charge to a central atom or to a surrounding atom, is it always favorable to have the 0 formal charge on the central atom?
is it always favorable to have the 0 formal charge on the central atom? No. It is in general more favorable to have the negative formal charge on the more electronegative atoms.
Hi, Professor! Since partial charges are related to the eletronegativity of two atoms. For two same atoms like Cl, there should not be any difference in their electronegativity so the two atoms won't have any partial charges. But in the textbook P76 the structures with partial charges do exist. Can you explain me why?
Since partial charges are related to the eletronegativity of two atoms. For two same atoms like Cl, there should not be any difference in their electronegativity so the two atoms won't have any partial charges. ClCl is a nonpolar bond. There is not permanent dipole moment in this bond. But in the textbook P76 the structures with partial charges do exist. There might be induced transient dipole moment in ClCl (London Dispersion Forces).
In chapter 2, is the answer in the answer book written backwards for problem #79? Polarizing power increases as the molecule get bigger correct?
Polarizing power increases as the molecule get bigger correct? No. Smaller, more highly charged cations have greater polarizing power
Hi, professor. In the textbook it is said that when we depict an electric dipole by an arrow, the arrowhead should point at the positive partial charged atoms. But in the course reading we are told it should point at the negative partial charged atoms. which one should we follow?
The arrow should point at the negative partial charged atoms
Professor Lavelle, Should we know how to use the Coulomb potential energy equation? And I'm confused on how lattice energies affect interactions between ions. Could you please clarify this? Thank you
Should we know how to use the Coulomb potential energy equation? Yes. But mostly it involves qualitative analysis. how lattice energies affect interactions between ions The lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound, i.e. the interaction between ions.
Hi Dr. Lavelle, I had a question regarding SelfTest 2.13B in the text book, which asks the following: In which of the compounds CaS and CaO do the bonds have greater covalent character? I would have thought that, being higher vertically on the periodic table, oxygen would be smaller, have a higher ionization energy, and higher electron affinity and therefore be more polarizing than sulfur, however the answer is CaS. Could you please explain why? Thank you.
In which of the compounds CaS and CaO do the bonds have greater covalent character? S^2 is larger (more polarizable) than O^2 =>the CaS bonds have greater covalent character than the CaO bonds.
Hi Professor, when we write the lewis structure of an ionic compound, should we draw the square bracket around the cation or the anion?
Yes, and you should also indicate the charge of the ion at the upper right corner of the structure.
Hi Professor, for dipoloe molecules how do you know which atoms are positive and which are negative?
The more electronegative atom will be partially negative. The atom with lower electronegativity will be partially positive.
Hi Professor, When reporting answers and units, both during a problem and in our answer, do we always need to specify what the units are measuring? For example, in a dilution problem, must we always report molarity as 9.85 mol KCl / L H₂0 ? Thanks
When reporting answers and units, both during a problem and in our answer, do we always need to specify what the units are measuring? Yes. For example, in a dilution problem, must we always report molarity as 9.85 mol KCl / L H₂0 ? You could report it as C(KCl)=9.85 mol/L. In addition, it is actually 9.85 mol KCl/1L of KCl aqueous solution, not 9.85 mol KCl / L H₂0.
For Chapter 2 homework problems, some deal with concepts and equations that weren't covered during lecture or aren't mentioned in the course reader. Are we still expected to know how to solve these questions?
Could you given me an example?
Hi Professor Lavelle! How do you find oxidation number? For example, NO, NO2? Thank you.
Please read Toolbox K.1 on page F80 of the text book. When assigning oxidation numbers of atoms in a compound, you just need to follow the rules listed in the toolbox. For example: NO: O is normally 2 => N should be +2; NO2: O is normally 2 => N should be +4
Hi Professor Lavelle! I have two questions. First, if the threshold frequency of a metal is given, to calculate the energy needed to remove an electron do we just use the formula E=hv? And second, what does it mean if there are underscores for n=1 and n=3 for a electron transition problem? It's in the form n(underscore 1)=1 and n(underscore 2)=3. Thanks!
First, if the threshold frequency of a metal is given, to calculate the energy needed to remove an electron do we just use the formula E=hv? Yes. what does it mean if there are underscores for n=1 and n=3 for a electron transition problem? It's in the form n(underscore 1)=1 and n(underscore 2)=3. It means before the transition: n_initial=n_1=1; after the transition: n_final=n_2=3
Hi Professor Lavelle, How to get the valenceshell configuration for Group 5 transition metals? Could you please specify it for me? Thanks!
The valence shell configuration for vanadium (group 5 period 4) is 3d^3 4s^2
Hello Dr. Lavelle. Just one quick question. For the Heisenberg Uncertainity Principle what is the delta v if its like 5 m/s plus or minus .1? Thanks!
delta v = 2*0.1 m/s = 0.2 m/s in this case.
Hi professor. I'm having trouble with Ch.2 #57a: Write the lewis structure of Chlorine monoxide. How do we know which atom has the free radical?
Oxygen has a higher electronegativity than chlorine. So the oxygen atom is less likely to be the radical than the chlorine atom.
Hi again, I'm having trouble with the following problem, would you be able to work through it? Thank you! 18. In 1.0 s, a 60 W bulb emits 11 J of energy in the form of infrared radiation (heat) of wavelength 1850 nm. What is the energy per photon of light emitted? How many photons of infrared radiation does the lamp generate in 1.0 s?
energy of one photon E0=hv=hc/wavelength Total energy emitted in 1s Etot=11J # of photons = Etot/E0
Hi Professor, I have a question about problem E.7 in the Fundamentals section. What formulas should be used to solve sections a and c? I have tried everything but cannot seem to obtain the correct answers given in the back of the book. Thank you.
Consider in 1 mol of MgSO4*7H2O, there are 11 mol of Oatoms, and 7 mol of H2O. i.e. n(O)=11*n(MgSO4*7H2O) n(H2O)=7*n(MgSO4*7H2O)
Hi Professor, I was reviewing the postmodule assessment on Molarity and Dilution of a Solution and two of the problems are the same (A solution is prepared by dissolving 55.1 g of KCl in approximately 75 mL of water and then adding water to a final volume of 125 mL. What is the molarity of KCl(aq) in this solution? ) I did the problem the first time and got a solution that was one of the possible answers, but it's not one of the choices the second time the problem is asked. There is only one possible answer shared by both problems and I can't seem to get that answer. Could you work through the problem? Thanks for your help, Grace
molarity of KCl(aq)＝55.1g/(74.5513g/mol)/(0.125L)=5.91 mol/L
Hi Professor, I have a question about problem 1.129 in our textbook. After consulting Figures 1.46(Atomic Radii) and 1.48(Ionic Radii) in Chpt. 1, I concluded that A=Cl, B=Na, C=Cl and D=Na+. However, the answers are the complete opposite, where A=Na, B=Cl...etc. How is this possible if the atomic radius of Na is larger than that of Cl, and if the ionic radius of Na+ is smaller than that of Cl? Thank you!
There is a typo in the solution. It should be: Na>Cl; Na+ < Cl.
Professor Lavelle, I have a difficulty doing question 85 from chapter 1 . Give the notation for the valenceshell configuration (including the outermost delectrons) of (b) Group 15/V elements; the answer is ns2np3. What about elements liek Ge, why don't we need (n1)d10. The question asks us to include the outermost delectrons ? (c) Group 5 transition metals; the answer is (n1)d5ns2, I don't know how to get this. (d) "coinage" metals(Cu, Ag, Au). For Au, do I need give notation for 4f14? Thanks!
What about elements liek Ge, why don't we need (n1)d10. For elements in the pblock, the (n1)d electrons are not the valenceshell electrons.
Hi Professor Lavelle. For the upcoming quiz, is there a convention on how many sig figs we should be using when inputting constants? For example, for Planck's constant, should we use the full value of 6.62608 x 10^34, or is 6.63 x 10^34 acceptable? Thank you.
You should use the constant given on the cover page of the quiz.
If a question requires a student to work with a chemical equation, but the chemical equation is not already provide, therefore the student must write the equation. I realized that usually the problem states the reactants, but how would we write the products? For instance, if a question asked: 5.62 grams of CaCl2 were reacted with 7.51g Na3PO4. The product will be calcium phosphate. I understand that you would write CaCl2 + Na3PO4 >_____ But I don't understand how you can create calcium phosphate, especially with Na and Cl part of the equation
Calcium phosphate is produced because it is a precipitate. how would we write the products? You will need to know how some very basic types of reactions take place. But normally the skeletal equation will be provided in the problem.
Hi Professor! I was just wondering what the minimum De Broglie wavelength required to be able to detect any wavelike properties.
Hi Lilian, We can measure wavelengths down to 1 femtometre = 1.0 x 10^−15 metres.
Hi Professor, If you ask us to write a balanced equation on the quiz, do you expect us to say if the reactants/products are in solid, liquid, or gas form? e.g. Nitrogen reacts with oxygen to form nitric oxide. N2 (g) + O2 (g) > 2NO (g) Thank you.
Hi Lawrence, Yes give the phases.
Hi Dr. Lavelle, I have been trying to retake the postmodule quizzes, but every time I click on the link, the only thing that comes up is the "thank you for taking this survey" page, and I can't seem to find a way to actually see the quiz again. Any suggestions? Thank you for your help.
Hi Grace, There was a setting that was changed today. You (and all students) should now be able to retake them as often as you want to prepare for Quiz 1, the midterm, and the final exam.
Professor Lavelle, I was curious to know if there was a difference between threshold frequency and photon frequency, and if so how would the energy of each be found. Thank you.
I assume you're referring to the photoelectric effect. Remember the equation for this is Energy(photon)= Threshold Energy Energy (emitted electron). You can see that by looking at the equation there is a difference between the threshold energy and photon energy (I interchange the frequency and energy because they are related by the equation E=hv). Specifically, the threshold energy is the minimum energy required to emit electrons from a metallic surface. The energy of the photon is simply the energy of the electromagnetic radiation being used to hit the metallic surface. This energy can take on whatever value, BUT until it reaches an energy greater than that of the threshold energy, no electrons will be emitted from the surface. It's also worth noting that the threshold energy is specific for every metal.
Question 1.21 of the textbook reads as follows, "A lamp rated at 32W emits violet light of wavelength 420nm. How many photons of violet light can the lamp generate in 2 seconds?" I worked the problem and cannot seem to get the answer, which is 1.4x10^20 photons.
First you have to calculate the energy from the lamp, which is simply 32J/s *2s = 64J. Next you are given the wavelength of the light that the lamp emits. You can calculate the energy corresponding to that wavelength (using E=hc/wavelength), which would give the the energy PER PHOTON of light. Once you have that, you can divide 64J by your answer which is in J/photon to give you the # of photons. Notice how the units work out.
Hi Prof. Lavelle, For limiting reactant problems, in the course reader you said to compare the moles of the reactants to see which one is the LR. But I noticed that you could hit 2 birds with 1 stone if you just calculated how many grams of the product is produced by each reactant. e.g. P4 + 6Cl2 > 4PCl3 So for the above problem, could I just go from moles of P4 to grams of 4PCl3 and moles of 6Cl2 to 4PCl3? This way I can find what the LR is (the smaller product) along with how many grams are actually produced. Thank you, Lawrence
You are correct, the method you use can be applied to simple examples such as the one you presented. This method is actually exemplified in the fundamentals section of limiting reactants in the textbook. It gives you the two ways to figure out those types of problems.
Hello Professor Lavelle! I need a little help for Quiz 1 Preparation Fall 2012 question 8. Question 8 reads: "For a hydrogen atom, find the wavelength for an e transition from n1=1 and n2=3." My question is that are we using the change in energy to solve for the change in wavelength (finding one wavelength), or are we using each energy we find to find its corresponding wavelength (finding two wavelengths in total)? Thank you so much!
Use the change in energy to find the wavelength that corresponds to that transition.
Hello Professor. I am having trouble with #7 on the Fall 2011 Quiz. Im not sure how to solve the second part of the question which says, "The work function for lithium is 4.6 e19 J. What is the maximum energy of the electrons emitted when light of 7.3 e14 Hz is used?" Thank you!
Remember that the equation of photoelectric effect is Energy(radiation)= Work Function + Energy (electrons emitted). You are given the work function and frequency of the radiation. So all you would have to do is convert the frequency to an energy (using E=hv) and rearrange the equation to solve for Enery of the electrons emitted, which is just: Energy (electrons)= Energy (radiation)  Work function.
Hi Professor Lavelle, I've been reading your answers for question 3 "Find the uncertainty in the position of a marble of mass 1.5 g given that it's speed is known to within + 0.55 m/s" in the Fall 2010 quiz and you said that there was an error in the solution and that the answer should be 6.40 x 10^32. However, I went to my TA, Devon, and another TA, William, and they both told me that in order to get the solution I would have to multiply 0.55 by two since it ranges from .55 to +.55. You also said that in the Fall 2011 quiz for question 5 "Your bowling ball of mass 4.02 kg rolls down a lane with a speed of 2.35 +0.1m/s. What is the minimum indeterminacy of its position?" that you would use 0.2m/s for your velocity since that would be the range. I'm not sure which way is correct. Could you clarify this issue since this applies to a problem on the actual graded quiz? Thank you.
I've already corrected my mistake in my original answers to these questions if you look at the "answered questions" section of VOH. Your TA is correct, I apologize for the mistake.
I'm confused about what it means to say that a photon has wavelength and frequency. Is one photon equivalent to one wave of radiation? How is a photon with a large wavelength different from one with a small wavelength?
The usage of the word photon illustrates that it has properties of waves and particles. A photon is explicitly described as a "packet" of electromagnetic radiation (remember photons are quantized). A photon is not just 1 wave of EM, but it depends on what kind of EM you're talking about; since they have different wavelengths and frequencies. Which brings us to your next question. Recall the equations E=hv and c=v*lambda. Rearranging the equations can give us something we've also seen: E=hc/lambda. You'll notice that a photon with a small wavelength will have a higher energy than that of one with a bigger wavelength. That's why photons are different depending on what EM you're talking about. Because they have different wavelengths and frequencies, which correspond to different energies.
Hi Professor, I'm having a hard time getting the uncertainty in position for this question. It reads "The hydrogen atom has a radius of approximately 0.05 nm. Assume that we know the position of an electron to an accuracy of 1 % of the hydrogen radius, calculate the uncertainty in the speed of the electron using the Heisenberg uncertainty principle". Thank You
1% of 0.05 nm is simply: 0.01*0.05nm = 5e4nm.
Dear Dr. Lavelle, Are we responsible for knowing the material in Chemical Equilibrium parts 1 and 2 for this week's quiz? Thank You!
As mentioned in lecture, only the fundamentals section and chapter 1 will be on the quiz. I'm not sure where you are getting chemical equilibrium from.
Hi Professor Lavelle, I had a question about problem 1.69 (b) in the textbook. The question asks to determine whether the electron configuration represents the ground state or an excited state of the atom given: (b) N: 1s ꜛꜜ, 2s ꜛꜜ, 2p ꜛꜜꜛ. The answer from the back of the textbook says this is the excited state, but I was wondering why that is so if the remaining 3 electrons occupy 3 different p subshells. Does the direction of the spin state play a role in determining whether an electron is in the excited state or the ground state?
You are correct. Although the 3 electrons occupy different orbitals, having one in the electrons be opposite spin would require some energy to accomplish this. You can imagine how this would not reflect the ground state because having it opposite to the other electrons in the orbital can cause some interactions that would result in some energy change. (when compared to the ground state)
Dear Prof. Lavelle, For Fall 2011 Quiz 1, Question 5 has an answer of 7 X 10^35 in the answer key. I'm pretty sure I'm doing the equation properly, but I keep getting 1.31 X 10^34. I've plugged in 4.02 for mass and 0.1 for delta v, and when I solve for delta X, I don't seem to get the correct answer. Am I doing something wrong?
****************ATTENTION TO ALL OF THE STUDENTS WHO HAVE HAD TROUBLE WITH THIS QUESTION AND QUESTION #3 OF THE 2010 QUIZ. I MADE A MISTAKE IN EXPLAINING THIS QUESTION BEFORE AND WILL CORRECT IT IN THIS ANSWER.*********************** Your set up for the problem is correct, with the exception of 1 thing. You'll notice in the problem that the uncertainty in velocity is plus OR minus 0.1 m/s. This means that your delta v should not be 0.1 but rather 2*0.1 to make up for the fact that the object travels either 0.1 m/s faster OR slower. You'll notice that divided your original answer by 2 will give you the correct one. THIS GOES FOR BOTH Fall 2011 #5 and Fall 2010 #3. I'm sorry for my previous incorrect answer.
Hi Professor Lavelle. I have a few questions regarding Quiz 1. Will we be tested on each Fundamental section in the book or only on the specific Fundamental sections assigned as homework? Also, what do we need to know about orbitals for the quiz?. Thanks.
As mentioned in lecture, the quiz will cover the fundamental sections listed on the recommended problems. But remember, oftentimes some fundamental sections you worked through overlap with other fundamental sections we may have not explicitly told you to do. So make sure you review any examples in which this is the case. Also, anything from Chapter 1 is fair game for the quiz; including orbitals.
For questions that involve an electron transitioning between levels (i.e. For a hydrogen atom find the wavelength for an e transition from n1=1 and n2=3), how many significant figures should we use for our final answer?
You answer should be based on the constant(s) you use in the problem (i.e. Rydberg's). Remember that integers (exact numbers) like 1,2,3, etc. aren't considered in sig fig calculations.
Dear Prof. Lavelle, Will we be deducted points on the quiz for having incorrect sig figs?
Yes, as well as for missing or incorrect units. Please also note that any deduction on sig figs or units will be for the FINAL ANSWER ONLY. We won't take off points for sig figs in intermediate steps of a multipart problem, for example. But make sure that you do show all your work when doing problems as credit will only be given when work is shown.
Hello Professor, For Chapter 1 #129 in the textbook, shouldn't the smaller atom (A) be Chlorine and the larger atom (B) be Sodium because the atomic radius decreases as you move across a period from left to right? Similarly, for the products, shouldn't the larger atom (C) be anion Cl and the smaller atom (D) be cation Na+ because cations are smaller than their parents and anions are larger than their parents? Thank you! Elizabeth
Yes, you are correct. There's a section on the webpage titled, "Solution Manual Errors." You'll find that this problem is included in that document. Excellent reasoning!
Hi Professor, I read one of your answers to a submitted question about Quiz Fall 2012 #3. I also am having trouble with this problem, which reads: "Find the uncertainty in the position of a marble of mass 1.5g given that it's speed is known to 0.55m/s." However, unlike the other student, my answer is close to the one in the workbook, it is 3.2x10^32 instead of x10^31. How do you do this? Am I correct in changing the 1.5g to 0.0015kg? Thank you!
You are correct in your unit conversion. However, like I mentioned before I get double what the answer is listed on the list. I'll break down the calculation. After rearranging the Heisenberg equation to find the uncertainty in position, we have: deltax= [(6.626e34Kg*m^2/s)/(4*pi*1.5e3kg*0.55m/s] The units should cancel out to give an answer of 6.4e32m. Feel free to show how you came up with the answer.
Professor Lavelle, for question number 3 in the Fall 2010 quiz 1 preparation in the workbook, my answer is double the answer key's. Also, I'm not sure how to do part 2 for that same question. Thank you!
I worked out that problem and there is probably an error in the answer key. So you answer for the first part would be correct. For the second part, recall that the De Broglie equation allows us to make a connection of wavelength to momentum: Lamba=h/p Recalling that p=m*v (mass * velocity), you should be able to find the wavelength since you are given velocity and mass.
This is not a homework question, but I took the class last year so I did not buy a course reader/practice quiz book for this year. When Quiz 1 comes around, am I allowed to get the questions from someone else's practice quiz book and turn them in on a separate sheet of paper?
At the very least, I would photocopy the corresponding pages from the purple book and answer them in the space provided. That way the grader would know what question you're working on.
Hi Professor Lavelle My question is: 6.40g of a compound was burned and produced 8.80g of CO2 and 7.20g of H20. Find the empirical formula of the compound.
Please see the similar answered question. It will direct you to Fundamental section M in the book, for an elaboration on these types of problems.
Hello Professor Lavelle! I have a quick question regarding the preparation quizzes in the purple workbook. The introduction in the beginning of the workbook states that we must give our workbooks to our TA's before quizzes in discussion section. I'm just a bit confused as to whether this means that we have to turn in our workbook during the discussion before the day of the quiz or if they are collected on the same day of the quiz. Thank you so much!
Hi Jasmine, Workbooks are handed in to your TA the same day as the quiz.
Hi I'm having trouble with #5 on the fall 2011 quiz one prep in the workbook. It says "Your bowling ball of mass 4.02 kg rolls down a lane with a speed of 2.35+ 0.1m/s, What is the minimum indeterminacy of its position?" I cant get the right answer I've tried multiple times. Thanks for the help
Hi Dilan, Make sure to use Delta v = 0.2 m/s because the velocity could be any value between 2.25 and 2.45 m/s.
Hello professor. During the lecture today, we used Heisenberg's Uncertainty principle to calculate the uncertainty of the velocity of an electron within an atom. In this example, why did we use the one dimensional diameter of an atom as opposed to an area, or a volume? Does delta(x) always have to be in one dimension?
Excellent question. The example given in class was introduced to first and foremost illustrate how one would use the uncertainty principle in a calculation. Second, the example was just a rough and simpler model to put the values into perspective. You're right in implying that confining the electron in 3 dimensions would be more realistic than 1. You can use a more realistic model to for this type of problem, but you would have to consider advanced problems using Schrodinger's Wave Funtions which is not expected of you in this class. When doing these types of problems, always just go with the uncertainties given to you. A lot of problems may use just 1 dimension to consider more simplistic models.
Hello Professor Lavelle, I am having trouble solving a question on Quiz Prep 1 (2010) regarding empirical formula. The question reads, " 6.40 g of a compound was burned in air and produced 8.80 g of CO2 and 7.20 g of H2O. Find the empirical formula." I know how to solve for empirical formula when it involves single elements such as, "....has a mass composition of C 39.43%, O 52.58%, and H 7.88% and a molar mass of 152.15 g/mol...." But i do not know how to solve for empirical formula when the elements are combined with other elements. Thank you!
Might I direct you to Fundamentals Section M (Pg. F100). This problem is a special example of empirical formula determination called Combustion Analysis. I encourage you to read about it. In general what occurs is that your unknown (typically a compound containing C, H, and O) will we combusted (i.e. O2 will be added), giving off only water and CO2 as your products. If Nitrogen is present in the unknown, N2 gas will also be emitted. Since only these 3 products are released, you can establish a relationship between the number of moles of CO2, H20, and N2, with respect to the number of moles of C, H, and N,respectively. Once you know the moles of the atoms, the rest is like solving a typical empirical formula problem.
Hi Professor Lavelle! For large everyday objects does Heisenberg's uncertainty (indeterminacy) principle play any measurable role? Which is the right answer: 1.no, the uncertainties in position, speed, and momentum of a stationary object are not noticeable or measurable or 2. No, the uncertainties in position, speed, and momentum of a moving object are not noticeable or measurable?
I think it's better to say that for everyday objects Heisenberg's Uncertainty Principle does not play any role because the uncertainties associated with larger objects are so small in general. What this means is that we are able to precisely estimate where an everyday object will be when given things such as velocity, momentum, etc. We can do this using classical physics. But small fast things like electrons don't obey classical physics, which is why quantum mechanics was developed! Recall the example in lecture regarding you walking into a supermarket. The photons that hit you when you walk through the door censors don't throw you off course right? So there is essentially no uncertainty in your path once you've established a velocity, momentum, etc.
Hi Professor Lavelle, I know that I am sounding irresponsible for asking this but do you have a lost purple workbook from one of last week's lectures? I may have lost it there and I have been looking for it everywhere. Please help Professor Lavelle and I am sorry for the inconvenience. Thank you.
Unless it is announced before lecture, you can assume nothing was found unfortunately.
Hi Professor Lavelle. Will we need to memorize nomenclature of compounds for quizzes and exams or will the molecular formula of compounds be given?. Thanks.
Nomenclature is something that we didn't go over in class, but is part of the fundamentals section. Therefore yes, you should be able to know how to name compounds. If you need a refresher, I'd look at the course reader (pgs. 1820 overall) and Fundamentals section D in the book.
Hi Professor Lavelle. Will we need to memorize nomenclature of compounds for quizzes and exams or will the molecular formula of compounds be given?. Thanks.
In time, you will need to know common nomenclature of compounds. For now, focus on the more importantly highlighted areas suggested to study.
Hi Professor, I have a question from the fourth module. Number 12 says: 5.00 g of KMnO4 is dissolved in a 150.00 mL flask of water. If 20.00 mL of this solution is removed and placed in a new 2nd 250.00 mL flask and filled with water, what is the concentration of the solution in the 2nd flask? Molar Masses: K (39.10 g/mol), Mn (54.94 g/mol), O (16.00 g/mol) I am not sure how to solve this because my attempt was completely wrong. The way I set up my equation was .24365(.13)= x(.02) .24365 mol/L was what I found the molarity of KMnO4 to be. And I used .13 L and .02 L because I subtracted the 2 mL that was taken out of the 15 mL so that left 13 mL in flask one and 2 mL in flask two.
First, I would double check your calculations for the molarity of KMnO4. If the molar mass of KMnO4 is The number of moles should be equal to 158.04 g/mol, 5.00g/158.04 g/mol =0.0316mol/0.150L= 0.211mol/L. In the second part, you're think of the right equation. The dilution equation is given as: CiVi=CfVf. Ci and Cf correspond to your initial and final concentration. Since you're solving for Cf, you can say: Cf=CiVi/Vf. Now you just need to know what to put for Vi and Vf. The problem states that you will take out 20.00ml of your initial concentration and dilute it to a total volume 250.00ml. Therefore, Vi=20.00ml and Vf=250.00ml. Remember that molarity is moles of solute divided by volume of the TOTAL solution (not just the solvent).
Hi professor, I'm not sure I understand the questions number 9 of the limiting reactant post module assessment. here it is: 9. How many moles of CO2(g) are produced when 1 kg of CaCO3(s) is used to neutralize an acid spill? The equation for the reaction at 1 atm and 25 degrees C is: CaCO3(s) + H2SO4(aq) > CaSO4(s) + CO2(g) + H2O(l) So does the fact that CaCO3 is used to neutralize the H2SO4 means that H2SO4 is the limiting reactant?
In this case, you are only given one piece of information, the amount of CaCO3 that you start with. So you must use this information and find how many moles of CO2 are produced (you don't know how much H2SO4 there is, so you cannot compare the two reactants to determine the limiting reactant). To determine moles of CO2, (1 kg CaCO3)*(1000 g CaCO3/1 kg CaCO3)*(1 mol CaCO3/ 100.09 g CaCO3)*(1 mol CO2/ 1 mol CaCO3)= 10 moles of CO2.
Hi Professor, I have a question from the fourth module. Number 12 says: 5.00 g of KMnO4 is dissolved in a 150.00 mL flask of water. If 20.00 mL of this solution is removed and placed in a new 2nd 250.00 mL flask and filled with water, what is the concentration of the solution in the 2nd flask? Molar Masses: K (39.10 g/mol), Mn (54.94 g/mol), O (16.00 g/mol) I am not sure how to solve this because my attempt was completely wrong. The way I set up my equation was .24365(.13)= x(.02) .24365 mol/L was what I found the molarity of KMnO4 to be. And I used .13 L and .02 L because I subtracted the 2 mL that was taken out of the 15 mL so that left 13 mL in flask one and 2 mL in flask two.
First, you need to find the molarity of the original solution, then the problem will be plugging into "M1V1=M2V2" to determine the concentration of the new diluted solution. Convert 5.00 g KMnO4 into moles by dividing by the molar mass, (5.00 g KMnO4)/(158.04 g/mol KMnO4)= 0.0316 mol KMnO4. Then divide by the volume of the solution made to find the concentration, (0.0316 mol KMnO4)/(.15000 L)= 0.211 mol/L KMnO4 solution. Now use the M1V1=M2V2 equation to solve for M2, the concentration of the new solution knowing that you use 20.00 mL of the original solution and have a total of 250.00 mL of the new solution. (0.211 mol/L KMnO4)*(.02000 L)= (x mol/L)*(0.25000 L). x mol/L= 0.0169 mol/L is the concentration of the new solution.
Dear Prof. Lavelle, Will we have to memorize which wavelengths correspond which range/type of electromagnetic radiation for the quiz and all subsequent exams?
Yes
Dear Professor Lavelle, I am having trouble solving question 17 on the Atomic Spectra Module Post Assessment. It states "The meter was defined in 1963 as 1,650,763.73 wavelengths of radiation emitted by krypton86 (it has since been redefined). What is the wavelength of this krypton86 radiation? To what region of the electromagnetic spectrum does this wavelength correspond (i.e. infrared, ultraviolet, xray, etc.)? What energy does one photon of this radiation have." Thank you
Hi Megan, In one meter there are 1,650,763.73 wavelengths of radiation. Therefore each wavelength is 1m/1,650,763.73 = 6.058 x 107 m (605.8 nm). The rest you can work out.
Hi Professor Lavelle. I have a minor sig fig question regarding Part D of question E.25 in the textbook. The question mentions 8.61 g of a compound is measured and Part D asks "what fraction of the total mass of the sample was due to oxygen?" The answer in the solutions manual is 0.3099 (4 sig figs), but I thought it should have been 0.310 (3 sig figs) to correspond to the 3 sig figs of 8.61 stated in the question. Would you be able to clarify this minor detail? Thank you!
I believe you are right, the final answer should be 3 sig figs, 0.310 g.
Hi Professor Lavelle, I had a question on number 23b on Chapter 1's homework. The problem states: "No electrons are emitted from the surface of the metal until the frequency of the radiation reaches 2.50 x 10^6 Hz. How much energy is required to remove the electron from the metal surface?" The solutions manual states that you should plug in the given frequency into E=hv, and E would be your answer. However, I thought the question was asking for the threshold energy Ö, and hence you would have to use the equation "Kinetic Energy = Energy of photon  threshold energy". Is the solutions manual correct or incorrect? Thank you very much for your time!
This question has been answered several times already in previous VOH questions. Please see below. To give you a shorthand answer, the threshold energy is the work function energy required to eject an electron from a metal surface. This frequency given is directly related to the energy required to eject an electron from a metal surface through E=hv.
Professor Lavelle, are we allowed to use calculators when having quizzes or exams? If we're allowed, then what type? Scientific calculator or graphical calculator? Thanks.
For quizzes and exams, you may only use nongraphing, nonprogrammable calculators.
Professor Lavelle, I have a clarifying question about the concept of quantum levels you discussed on Wednesday. I know that when light is absorbed, the electrons move up levels, and when light is emitted the electrons move down. But, does this also mean the energy level increases as the quantum levels rise and the electrons get further from the nucleus? I just wanted to make sure I am understanding the concept correctly. Thank you.
For this course, the energy levels remain the same despite where the electrons are moving to/from. Electrons are moving to higher or lower energy levels, which themselves remain at the same energy value regardless if they are vacant or not.
Hello professor Lavelle, I have a question about photoelectric effect. In the last lecture, you mentioned that increasing the intensity of light will increase the number of electrons ejected from the metal surface. My question is, does increasing the frequency of light increase the intensity of light, which then will result in increased number of electrons ejected?
Increasing the intensity of light increases the energy of the light, which is equivalent to saying increasing the intensity of light increases the frequency of light and decreases the wavelength of the light. By increasing the frequency and decreasing the wavelength of light, more photons during any period of time can travel. For the photoelectric effect, you can think of photons as packets of energy; one photon ejects one electron. But after increasing the intensity of light, more photons are able to eject more electrons for any given period of time.
Hi Professor! In 1.23c, the question reads: "The velocity of an electron that is emitted from a metallic surface by a photon is 3.6x10^3 km/s. What is the wavelength of the radiation that caused photoejection of the electron?" From part a and b, we have found that the wavelength of the ejected electron is 2.02x10^10 m and that the energy required to remove the electron from the metal surface is 1.66x 10^17 J. The problem also states that the frequency of the radiation is 2.50x 10^16 Hz. I don't understand why I can not use the equation c= wavelength x frequency to solve for the wavelength, using the frequency 2.50 x 1^16. Thank you.
In this case, a photon hits a metal surface and not only ejects an electron but also makes it "fly" off with a certain velocity. This means that the energy of the incoming photon exceeded the energy required to eject the electron. The problem states that no electrons can be ejected until the photon has a frequency of AT LEAST 2.50 x10^16 Hz (this can be converted to the work function of the metal by using the equation E=hv). But we already know the photon had more energy than that because the electron was not only ejected but went off with some velocity. In this case, we need to use the conservation of energy equation, E(incoming photon)= (phi, work function of the metal)+(kinetic energy of the outgoing electron, 1/2mv^2, where m is the mass of the electron and v is the velocity in m/sec) in order to find the total energy of the photon, and then we can determine its wavelength. By using the work function energy you determined in part b and the velocity of the electron stated in the problem, you can plug in E(incoming photon)= (1.66 x10^17J) + (1/2)*(9.10939 x10^31 kg)*(3.6 x10^6 m/sec)^2 = 2.25 x10^17 J. That is the energy of the incoming photon. Now we can go back to our energy of light equation to solve for frequency, E=hv, or v= (2.25 x1017 J)/(6.626 x10^34 J*sec)= 3.3957 x10^16 Hz. And lastly solve for wavelength using the speed of light equation, (lambda)=(3.0 x10^8 m/sec)/(3.3957 x10^16 Hz)= 8.8 x10^9 m.
Hi Professor, I was wondering if we need to know/memorize unit conversion constants. For example, there is a problem in the textbook that gives the energy in the units eV and to solve it we need to convert it to J. How would we know what to multiply it by to convert the units?
You do not need to memorize constants, including the conversion of eV to J. Other constants like Avogadro's Constant, Planck's Constant, Rydberg Constant, Speed of Light Constant, etc. will be given to you on any quiz or exam. Also, the mass of an electron, the mass of a proton, etc. will also be supplied to you on any quiz or exam. Furthermore, most equations you need for calculations are also given to you for quizzes/exams. The purpose of this course is to understand the material and be able to manipulate equations/constants to find useful information, not memorize them.
Hi Professor Lavelle, I have a question regarding question 2 in the first Quiz 1 Preparation Quiz in the work book. After balancing the eqn 2Al(s)+6HCl(aq)>2AlCl^(3)(aq)+3H^(2)(g) the question reads as follows: If you had 5.43 g of Al and 7.80 g of hydrogen chloride how many grams of AlCl^(3) would you get? I then proceeded to find the theoretical yield of AlCl^(3) by calculating the limiting reactant (which I found to be HCl), ultimately ending by using the equation m(AlCl^(3))=nM, where n=the amount of moles of AlCl^(3) that 7.80 g HCl would be able to make. However, the answer that I got was far larger than the 9.51 g answer provided in the solutions. Where did I go wrong?
I'm not quite sure about your explanation of how to find the amount of AlCl3 formed but here is how you should do it: You determined the limiting reactant correctly, it is HCl. The limiting reactant is found by converting all the reactants to moles and dividing by their stoichiometric coefficients, whatever is the least amount is the limiting reactant. In order to find the amount of any product formed, you start with the amount of the limiting reactant formed, in this case 7.80 g HCl. The calculation should be as follows: (7.80 g HCl)*(1 mol HCl/36.45 g HCl)*(2 mol AlCl3/6 mol HCl)*(133.33 g AlCl3/1 mol AlCl3)= 9.51 g AlCl3.
Hi Dr. Lavelle, I need some help with number 23 from the chapter one homework please. The question reads: The velocity of an electron that is emitted from a metallic surface b a photon is 3.6x10^3 km.sec^1. (a) What is the wavelength of the ejected electron. I learned that I need to use the de Broglie's relationship to solve this, however, I don't know how to find "m" for that formula.
Indeed the de Broglie relationship equation is (lambda)=(h, Planck's constant)/((m, mass of the particle)*(velocity of the particle). In this case, the particle of interest is the electron, and the mass of an electron is a constant value independent of the element it belongs to, and is written on the back cover of your book (and will also be given to you on any quiz/exam). The mass of an electron is 9.10938 x10^31 kg.
Hello Professor Lavelle! I have a question for the Chapter 1 review for homework on number 17. It reads, "the gamma ray photons emitted by the nuclear decay of a technetium99 atom used in radiopharmeceuticals have an energy of 140.511 keV.Calculate the wavelength of these gamma rays." When the book says 140.511 keV, would it mean I have to use the formula hv(phi)and look for the velocity of a yray photon or the velocity of the technetium99? If neitherof the options are correct, can you me what conversions (if any)to solve this problem? Thank you.
An eV is just another unit of energy and can be converted to Joules. (The conversion is on the back cover of your book and also will be provided on a quiz if need be). The conversion is 1eV= 1.60218 x10^19J. Remember, in this case you are talking about a gamma ray photon, which does not have mass. The photoelectric effect here does not apply and you only need to use the light equations you have learned, namely c=(lambda)*(v) and E=hv (Remember, v here is frequency in 1/sec, NOT velocity). After converting eV to Joules, (140.511 keV)*(1000eV/1keV)*(1.60218 x10^19J)= 2.25124 x10^14 Joules, you can this energy into E=hv to determine the frequency of the photon, namely (2.25124 x10^14J)/(6.62608 x10^34 J*sec)= 3.39758 x10^19 1/sec. Finally, determine the wavelength from this frequency by plugging into the speed of light equation, namely (3.0 x10^8 m/sec)/(3.39758 x10^19 1/sec)= 8.82981 x10^12 m. Satisfactorily, this wavelength corresponds to gamma ray radiation!
What is the difference between a chemical formula and a molecular formula?
Many people use these terms interchangeably, although technically a chemical formula is a more general term that can be applied to inorganic compounds, acids/bases, salts, molecules, etc. and a molecular formula should be applied only to molecules (covalently bonded materials).
Hi Professor, in the course reader pg 113, you found the equilibrium constant for the gasses by using using the ratio of the concentrations. But on the next page, you found the equilibrium constant by using the partial pressure of the gasses. When do we use Kc or Kp when dealing with gasses? Thanks
Hi Anna, We are not covering this section for another 6 weeks. To answer your question, which we'll also cover in class, for gas phase reactions one can use Kp or Kc. Which one we use depends on how the information was given (partial pressures of the gasses or concentrations).
Dear Professor Lavelle, I'm a bit confused about the difference between atomic emission spectroscopy and atomic absorption spectroscopy. Could you help explain the difference? Thank you so much.
Hi Carlos, Atomic emission spectroscopy is when the emitted light from the excited atoms is measured. Atomic absorption spectroscopy is when the light absorbed by atoms in the groundstate is measured.
Hi Dr. Lavelle, I realized I don't have the Purple Workbook for the class (I'm currently using my friend's course reader from last year). Is it necessary that I buy both the course reader / workbook from the bookstore in Westwood? Or do you know if I can just purchase the workbook itself? Thank you.
You can do whatever you would like, although it is recommended you have both the course reader and workbook. Also, I do not think you can buy them separately.
Hello Professor Lavelle, I have encountered one problem both as number 4 in the self test section of the workbook as well as number 3 in the Post test of the Empirical and Molecular Formula video and still cannot solve it. 339.20 g of Cobalt metal is reacted with Fluorine gas to produce a compound with a mass of 996.08 grams. What is the empirical formula of this new compound? I know that the moles of the reactants and the products must be equal as well as the masses of both sides, but I still can't figure out how to go about solving this problem. Thank you.
The total mass of the compound is 996.08 grams. And 339.20 grams of the compound is Cobalt, meaning the remaining 656.88 grams is Fluorine. Next you need to find the mass % of each element in the compound. (339.20 g Co)/(996.08 g total)= 34.05 % Cobalt. (656.88 g F)/(996.08 g total)= 65.95 % Fluorine. Now assuming 100 g sample, convert the elements to moles. (34.05 g Co)/(58.93 g/mol Co)= 0.5778 mol Co. And, (65.98 g F)/(19 g/mol F)= 3.473 mol F. Next, divide all elements by the smallest amount of moles, in this case, divide both elements by the moles of cobalt. In this case you get 1 mole of cobalt to 6 moles of fluorine, and therefore the empirical formula is CoF6.
Hello professor, I had a question about the kinetic energy of an electron removed from a metal. Does the energy input from the photon only have to match the required energy removal amount or does it have to exceed it in order for the electron to be FULLY removed? I ask this question because I thought that in order for the electron to reflect up to the "detector" (in your example of the photoelectric effect) the electron must have kinetic energy.
The energy input of the photon only has to match the required energy (or work function) necessary to remove an electron.
I have a question regarding the first worked example in the reader, which you went over in class. How is it possible that an electron with zero kinetic energy can be removed from the metal surface? I thought that would mean that E in the E=hv equation or the E(k) equation would equal zero, and therefore, the electron would not be able to be removed?
I think you are getting confused with the equation, E =(h, Planck's constant)*(v, FREQUENCY of the photon, in Hz), which describes the energy of a given photon, and the kinetic energy equation, k.e. = (1/2)*(mass of a particle, in this case an electron)*(VELOCITY, in m/sec)^2. The "v" in the two equations are completely different. In this particular example, the incoming photon has energy, while the removed electron has no kinetic energy.
Hi Professor Lavelle. For problem #23 in the textbook homework, I am having trouble understanding part A and I have a quick sig fig question dealing with part C. The problem's given information is the velocity of the ejected electron, which is 3.6 x 10^3 km/s. 1) Part C asks, "What is the wavelength of the radiation that caused photoejection of the electron?" How is this question different than Part A, which asks for "the wavelength of the ejected electron"? I was thinking that when calculating the wavelength of the ejected electron, you would use the E(photon) = Ek + E(remove e), but it is assumed that this equation is used to solve for part C instead. Would you be able to clarify why I cannot use this equation to solve for part A? Why is the de Broglie equation a more suitable formula to solve part A? 2) Additionally, while I was solving part C, I set up the equation, wavelength = (hc)/E(photon), and came up with a minor sig fig question. When calculating the E(photon) to plug back into this equation, I got E(photon) = (5.9 x 10^18 J) + (1.66 x 10^17) = Kinetic energy + the work function (which are the same numbers that the solutions manual got up to this point). But when I added the two numbers I got 2.2 x 10^17 (1 decimal place after adding), whereas the solutions manual got 2.25 x 10^17 (2 decimal places). I thought that when you add two numbers, it is appropriate to have the sum take on the least number of decimal places, and thus I thought the sum would take on 1 decimal place instead of 2. Would you be able to clarify this small significant figure question? I'm struggling a bit to understand the minor details of sig figs and want to make sure I'm not thinking about it in the wrong way. Thank you so much, Professor!
For an electron (a particle of matter) with a given velocity, the de Broglie equation can be used, namely lambda, in meters = (h, Planck's constant)/((mass of the particle, in kg)*(velocity of the particle, in m/sec)). So for part a, to determine the electron's wavelength, plug in lambda, in meters =(6.626x10^34 J*sec)/((9.10938x10^31 kg)*(3.6x10^6 m/sec)) to get 2.0x10^10 m. For part a, you are calculating the wavelength of the outgoing electron that was ejected. You use the de Broglie equation because you are determining the wavelength of a PARTICLE. Part c, on the other hand, asks you to calculate the wavelength of the incoming photon that ejected the electron. In this case, you will first need to combine the energy it took to eject the electron, the work function (the answer from part b) and the energy used to make the electron travel with a certain velocity (by using the kinetic energy equation, namely k.e. = (1/2)*(mass of the electron, in kg)*(velocity of the electron, in m/sec)^2). This is a calculation of the total energy of the photon. After you obtain this value in Joules, you can use the equation E = ((h, Planck's constant)*(c, Speed of Light constant))/(lamda, wavlength, in meters) and solve for lambda. lambda, in meters = ((h, Planck's constant)*(c, Speed of Light constant))/(energy of the incoming photon, in Joules). You are correct with your way of thinking of sig figs. In this case, you can see that there energy value of 2.25x10^17 Joules is not a final answer, and if it was a final answer it would be 2.2x10^17 J based on the addition rules of sig figs. Although they plug in 2.25x1017 Joules to determine the wavelength of the incoming photon, they do indeed make a final answer with only two sig figs (or one sig fig after the decimal point). This is because it is best to perform the rounding off at the end of a problem.
Hello Professor, In many of the homework problems the questions tell us to write the balanced equation for the reaction they are describing in the question. For example, " Solid boron can be extracted from solid boron oxide by reaction with magnesium metal at a high temperature. A second product is solid magnesium oxide. Write a balanced equation for the reaction. " On the quizzes and tests, are we going to need to know how to write the equations, or are we just going to be given the equations? Thanks, Madison
In a later chapter, you will learn how to name chemical formulas based on their names and oxidation states. Although it's good to start getting familiar with these types of problems, I wouldn't worry too much about that and I would focus on performing the actual balancing of the reaction. Also, keep an eye out for key words, like "A" reacts with "B" to form "C" in which case you would need to know what chemicals are the reactants and which are the products in order to set up a chemical equation.
I was wondering why the answer to question E1 in the textbook is not doubled because the radius of the atoms is 144 pm and they are strung side by side so we should be using the diameter.
Yes, see below as this question was previously answered.
Hi professor Lavelle, I was wondering if this equation, c=¥ë¥í describes photoelectric effect :)
Please rewrite this question, it cannot be read. Thanks.
For the suggested readings in Chapter 1. Are we supposed to know the many different laws like the StefanBoltzmann Law and Wien's Law? Or are we supposed to follow along with the outline and the suggested materials. Thanks!
Follow along with the outline and suggested materials of course, yet all of Chapter 1 is suggested to be read and understood.
Hello, for number five on the chapter one practice problems I am having difficulty calculating the wavelength when given the energy of the photon (5.768x10^19 J) as well as its frequency (8.7x10^14 Hz). I used the formula E=hc/(wavelength) and plugged in the energy of the photon for E yet cannot seem to get a correct answer. Can you tell me what I am doing wrong along with how the units work in this problem? Oh and I apologize for sending two unfinished questions, my computer kept sending them before i had finished typing. Thanks, Kelsey
For the first part of question 1.5, when given a frequency of 8.7x10^14 Hz and trying to determine the wavelength, you want to use the speed of light equation, namely c=(lambda, wavelength, in meters)*(frequency, in Hz, 1/sec). Solving for lambda/wavelength, in meters, you have lambda=(3.0x10^8 m/sec)/(8.7x10^14 Hz) and get a result in meters, specifically 345 nm, therefore 340 nm when you take sig figs into account. Then, to determine the energy of this radiation, you want to use the equation E=(h, Planck's Constant)*(frequency, v, in Hz). Your answer will be in Joules. You can also substitute for frequency using the speed of light equation and plug in E= ((h, Planck's constant)*(c, Speed of Light constant))/(lamda, wavelength, in meters) and still will get an answer in Joules, specifically 5.8x10^19 J. If you have the frequency and energy of the photon, and are trying to find the wavelength of the photon, it is easiest to simply use the frequency with the speed of light equation. You can still use the energy equation with frequency and solve for wavelength, although this is the longer way of solving for wavelength. Namely, lambda, in meters = ((h, Planck's constant)*(c, Speed of Light constant))/(energy of the photon, in Joules). It is important to be able to manipulate these two equations and always writing the units in your calculations.
Hi, In your lectures you tell us that in chemical equilibrium, if K > 10^3, then the products are favored. How come in the Post assessments, it seems that if K>1, the products are favored?
Please be more specific with your question.
When light has a higher intensity, does it mean that the number of photons increase? Or does it mean that the light has bigger wavelengths?
When a certain type of light has higher intensity than another type of light, it means it has SHORTER wavelengths, LARGER frequencies, and LARGER energy.
Hi Dr. Lavelle, I just wanted to clarify something regarding the outline of this week. Are the last 4 videos that you want us to watch topics that we we will be going over this week or topics you expect us to know/want us to review?
Depending on your previous Chemistry training, these topics may or may not be new. Regardless, they are topics that will be discussed as Chapter 1 is discussed and are encouraged to be completed for practice and familiarity.
Hi Dr. Lavelle, for the assigned problems in the beginning of each chapter in the course reader, do we find them at the end of the chapter in the textbook labeled exercises?
Yes
There's a Balancing Equation question in the course reader that I was unsure about. It says the answer can be: 2C6H7N + 31/2O2 > 12CO2 + 7H2O + N2 OR 4C6H7N + 31O2 > 24CO2 + 14H2O + 2N2 Are both answers correct? Isn't the second answer the only acceptable one because you can't have a fraction as a coefficient? Thanks
Both answers are correct and have identical meaning. You can have a fraction as a stoichiometric coefficient, although usually it makes calculations simpler to work with whole numbers.
Hello Professor Lavelle. I was reading the textbook and trying to work out the review problems listed in the syllabus and am having trouble understanding the concept of formula units. How would I, for example, find the formula units in 25.92mg of HF? What purpose does knowing the formula units of a certain compound serve?
Formula units are the smallest unit of an ionic compound. For the purpose of this course, the formula unit of any salt (ionic compound) will be given to you and you may have to determine the amount of formula units in a certain amount of substance. For your example, the formula unit of HF is HF. Starting with 25.92 mg of HF, you first convert the mass of HF in (mg) to (g) of HF, then convert (g) of HF to moles of HF by dividing by its molar mass, 20.01 g/mol, then convert moles of HF to formula units of HF by multiplying by Avogadro's constant, 6.02214E23 formula units/mol. Formula units are another way of counting how much matter is in a certain substance, similar to counting atoms, molecules, moles, etc.
Hello professor, I was reading through chapter one, and noticed table 1.1 depicting the frequencies and wavelength and energies of different types of electromagnetic radiation. I was wondering if we are responsible for memorizing these values, or if this information will be given on any examinations. Thank you
You do not need to memorize exact values for each type of electromagnetic radiation. It is most important to remember the types of electromagnetic radiation listed in Table 1.1 and the typical range of wavelengths for each type. Figure 1.9 on Page 6 is a good reference for the trends you want to know. In this way, you should be able to complete a problem such as 1.5 from the homework and determine the type of radiation depending on the wavelength.
Hi Dr. Lavelle, I'm a bit confused as to how we should be pacing ourselves through the work. As I understand it, we should have the second set of four modules done by the end of week two, but I'm not sure what the time frame is for the "Quantum World" Chapter 1 reading and questions. Should those also be done by the end of Week 2? I was also wondering how you recommend that we prepare for class I would usually review the notes (say, in the course reader or solution manual) but I'm not quite sure how to know what to study before each lecture. Thank you for your help, Grace
Hi Grace, My lecture notes are in the course reader and we will typically cover 4 pages per lecture so you can see where we are and what we will be doing. Since my lectures notes for the first section (Chapet 1) is 26 pages that is about 67 lectrures or approximately 2 weeks. See what we will cover in the next lecture by looking at my lecture notes and do background reading of the same material from the textbook. This will be great class preparation. You're welcome.
Hello professor Lavelle, I have a question about the third video module, "Limiting Reactant Calculations". Number 11 reads: 11. According to the following equation, 0.750 g of C6H9Cl3 is mixed with 1.000 kg of AgNO3 in a flask of water. A white solid, AgCl, completely precipitates out. What is the mass of AgCl produced? C6H9Cl3 + 3AgNO3 > AgCl + C6H9(NO3)3. Molar Mass: C6H9Cl3 (187.50 g/mol), 3AgNO3 (169.88 g/mol), AgCl (143.32 g/mol) I am not sure how to go about this problem..can you walk me through it step by step please?
Hi Sam, 1. Balance the equation: C6H9Cl3 + 3AgNO3 > 3AgCl + C6H9(NO3)3 Balanced chemical equation shows a 1:3 C6H9Cl3:AgNO3 molar ratio. 2. Convert 0.750 g of C6H9Cl3 to moles. 3. Convert 1.000 kg of AgNO3 to moles. 4. Are the moles in steps 2 and 3 in a 1:3 ratio? If not, which reactant is needed to have a 1:3 ratio, that is the reactant which is the limiting reactant. See example in Course Reader.
Hi Professor Lavelle, I was wondering if you can post the answers to all the postassessment questions (for assessments 18) so that we check if we got the right answer or not.
Hi Jessica, Students need to ask specific questions to get specific answers :) or maybe :(
Hi, Professor Lavelle! I have a question about the Fundamentals section in the textbook, page F44, E1. The field of nanotechnology offers some intriguing possibilities, such as the creation of fibers one atom wide. Suppose you were able to string together 1 mol Ag atoms, each of radius 144pm, into one of these fibers by encapsulating them in carbon nanotubes. How long would the fiber extend? I think we should multiply the given radius by 2 to get the diameter of an atom, and then we can get the whole length of the string. However, in the standard solution it just uses the radius as the size of an atom. The atomic radius is only the half distance from the centers of two neighboring atoms. Can you explain that for me? Thank you very much!
Hi Mengdi, I agree with you. Multiply the given radius by 2 to get the diameter of the atom.
Hi Professor Lavelle. For #18 (Part B) on the PostModule Assessment for the Photoelectric Effect, I am having trouble calculating the energy required to remove an electron from a sodium atom. The given information includes the ejected electron's velocity (6.61 x 10^5 m/s) and sodium's work function (150.6 kJ/mol). How would I solve this problem? Thank you!
Hi Donya, We will cover this in detail tomorrow in Monday's class. 1) Calculate the electron's kinetic energy using E(k) = 1/2 mv^2. This is the 'excess' energy. 2) electron's kinetic energy + sodium's work function (per atom) = energy of incoming photon. 3) However your question above asks for "the energy required to remove an electron from a sodium atom" which is simply the work function (per atom).
Hello Dr. Lavelle I was wondering about the homework. Will that be posted or is it just the textbook pages and problem numbers listed in the syllabus? How do we know what readings and problems to do when? Thanks so much, Shva
Hi Shiva, "I was wondering about the homework. Will that be posted or is it just the textbook pages and problem numbers listed in the syllabus?" The Syllabus is the Syllabus ... and it has about 200 HW problems (~20/week or ~3 problems every day of the week for the entire quarter). The homework is more than enough. Although there are also 100 plus pages of past quizzes, midterm, and final exams to work through and review. That is more than enough HW and problems. Do the readings and problems as we work through the material in class. You have my lecture notes in the course reader so you can also see what we will cover in the next lecture which is approximately 4 pages per lecture.
Hi Professor Lavelle. For module #3 in the post assesstment #7 it says "determine the limiting reagent if 21.4 g NH3 is reacted with 42.5 g of O2." 4NH3(g) + 5O2(g) > 4NO(g) + 6H2O(g) I'm really confused as to how to determine the limiting reactant since the ratio of NH3 to O2 is 4:5 but there are less moles of the O2 (1.33 mol vs 1.43 moles of NH3). Thank you so much!
Hi Thu, In the balanced chemical equation NH3 to O2 is 4:5. You calculated NH3 to O2 to be 1.43 to 1.33. Since the moles of O2 are less than the moles of NH3 the O2 is the limiting reactant ...
In the class today we talked about the wavelength of visible light. It is written on the material that the wavelength of visible light is approximately equal to 500nm, but in the following lines it is written that EM radiation from 400nm to 700nm, which human eye can detect, is the wavelength of visible light. So i am quite confused with these three numbers,why you give us an approximate number of 500nm? I think each visible light from red to violet has its own wavelength, which is constant, so there shouldn't be an approximate value.
Hi Mengdi, 400nm to 700nm is the range of wavelegths for visible light. However if someone asked you for a typical wavelength for visible light then 500nm is an easy/good number to remember.
Hi Professor Lavelle, For question #22 in postassessment, would C be 1 mol, since the limiting reagent is A and it is 1 mol?
Hi Hyo, Which assessment?
Hi Professor Lavelle. For question 9 on postmodule assessment #3, how am I supposed to know which is the limiting reagent if the question only gives out the mass of CaCO3? Don't we also need to know the mass of H2SO4 in order to figure out which one is the limiting reagent? Thank you!
Hi Hyo, Given the mass of one reactant it is assumed the reaction will be limited by it. That is, the other reactants are in excess.
Hello Professor Lavelle! Is there any written homework to be turned in on the first day of class, or do we only need to complete the assessments?
Hi Shannon, As described in the Syllabus homework is not handed in. The first 4 online assessments are good review of high school and fundamental chemistry concepts. The first 8 online assessments are an excellent resource in preparing for quiz 1.
why is it that after completing there always pops out a "surveyMonkey" add? we dont have to do those surveys right?
Hi Ernesto, Only do chemistry pre and postassessments (and the survey at the end of each postassessment). Anything else to do with Survey Monkey adds, etc., please ignore. Thanks.
Professor Lavelle, I'm confused on number 10 on the first postmodule assesstment. Do I still divide the percentages by the molar mass even though the molar mass is in g/mol instead of g?
Hi Jamie, Hard to follow your question. Molar mass is always g/mol. See page 26 in the course reader to see how to determine molecular formulas.
Hi Professor Lavelle, I have a question from the second assessment test (#9). How would we know that the moles of O2 is 15 if it wasn't a multiple choice question? Is it just always whatever coefficient ends up being in front of the molecule after balancing the equation?
Hi Jasmine, You answered your own question. Balance the chemical equation and the coefficients in front of the molecules are the molar ratios.
Hi, professor, i had a question after trying to solve post assessment for balancing chemical equations. The question asks about the net number of moles of gas produced in this chemical equation : 4C4H10(g) + 26O2(g) > 16CO2(g) + 20H2O(g) can you teach me how to do this problem?? thank you!
Hi Soo, Net means overall. Net change = moles gas product  moles gas reactant = 36  30 = +6
Hi Professor, In the first module, you talked about vitamin C and calculating the molar mass. What we are trying to find is what one mol of these will weigh? You calculate 3 multiplied by 12.01 plus 4 multiplied by 1.08 plus 3 multiplied by 16, which gives you 88.06. I wanted to ask where did you get the figures 12.01, 1.08, and 16 from?
Hi Crystal, They are the atomic molar masses from the periodic table.
Hello Professor Lavelle, I bought the chemistry books from a girl who took your class last year. Now I realize that it is the fourth edition of the chemical principles, and not the required fifth. Is this okay, can I still use this book? Thanks.
Hi Madison, It is okay but you'll need to check chapter numbers and homework problems and see how they correlate between the different editions.
Hi Professor Lavelle, I am wondering about the MDT exam that is taking place tomorrow. I took the math placement exam at my orientation during August, and was placed in Math 1. Should I still take that math exam that is being offered tomorrow?
Hi Amanda, Please ask the Math department. I think they let students take the exam more than once and then use the improved score to determine what Math class you take. But check with Math Sept.
Dear Professor Lavelle, I was wondering if you would recommend purchasing the course reader for Fall 2012 or could I use the 2010 course reader? I know some professors keep their course readers the same, but I wanted to be sure I was properly prepared for the class. Thanks!
Hi Linsay, You need the Fall 2012 course reader.
Hello Professor Lavelle, I just finished postassessment for limiting reactant calculations and had a problem for question Can a reaction have two limiting reactants? Justify your answer.I don't know if two reactants completely react with each other and have no excess can be considered as two limiting reactants. If not, then for a chemical reaction, can there be no limiting reactant or there must
Hi Eve, There can be only one limiting reactant in a reaction.
Hi Professor Lavelle! I just watched the video on the Empirical Formula but I got confused with one little step. Why was it that after figuring out that the ration of C:H:O was 1.00:1.33:1.00, you multiplied that by 3? Thank you!
Hi Jasmine, Atoms need to be whole numbers. Can't have a molecule with 1.33 of an atom. Always multiply by the smallest possible number, in this example, 3, to get whole numbers. (1.00:1.33:1.00) x 3 = 3:4:3
Hello there Professor! I seem to be having the same technological issue at Vatche. I have a Windows laptop & already have Windows Media Player installed, but every time I try to view a video module I get the same "install Windows Media Player plugin" message. I was wondering if perhaps this might have to do with the browser I am using (Google Chrome). If so, then what is the best browser to use to view the modules? Thanks!
Hi Jhenevieve, The best browser to use is Internet Explorer (IE).
Professor Lavelle, Do we have to do the "surveys" at the end of the assessments?
Hi Vatche, YES! Thank you.
Could you work through the following problem? Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 s1 before it can emit an electron from its surface. Answer the following two questions. B. If molybdenum is irradiated with 194 nm light, what is the maximum possible kinetic energy of the emitted electrons? Thank you!
Hi Grace, In part A you calculated the minimum energy (i.e., the work function per atom). In part B convert the 194 nm light to energy. This is the energy per incoming photon. Subtract the energy required to remove an e (i.e., the work function per atom) from the photon energy, and the remaining energy is maximum possible kinetic energy of the emitted electron. Your Welcome!
In regards to this question: Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 105 m.s1. The work function for sodium is 150.6 kJ.mol1. What is the frequency of the incident light on the sodium metal surface? I don't understand how to cancel out the "mol" in the figure 150.6 xJ.mol^1. Thank you!
Hi Grace, Good that you are thinking about this which I will discuss in class. It is important that one thinks about the photoelectric experiment as 1e interacting with 1 atom at the metal surface. Therefore the work function (kJ.mol1) must be divided by Avogadro's Number to get the energy per atom (not per mole).
In the assessment for "Empirical and Molecular Formulas" we are given: 3. 339.20 g of a cobalt metal is reacted with fluorine gas to produce a compound with a molar mass of 996.08 g/mol. What is the empirical formula of the new compound? Molar Mass: Co (58.93 g/mol); F (19.00 g/mol) Is it safe to assume that when we are given "a compound was produced" to assume that one mole of the compound and therefore 996.08 g was produced. When I make that assumption I find that the empirical formula is CoF6. Otherwise, I don't know how else to solve the problem.
Hi Lawrence, Correct.
Mr.Lavelle is it necessary that we attend the first discussion on thursday before class?
Hi Africa, Yes.
Hi Professor Lavelle, I was doing the Limiting Reactant Calculation postassessment and I'm having trouble with number 23. "Consider the unbalanced reaction: PCl3+ H2O >HCl + H3PO3 What mass of HCl is produced by the reaction of 23.6g of PCl3?" I believe there is a 1:3 ratio, but I don't know what else to do. If you can please help me out, it would be great! Thanks!
Hi Cindy, The balanced reaction is: PCl3 + 3H2O > 3HCl + H3PO3 Then convert 23.6g of PCl3 to moles PCl3. You are correct the PCl3:HCl ratio is 1:3. Therefore multiply your calculated moles PCl3 by 3 to get moles HCl produced. Then convert moles HCl to grams.
Hello Professor Lavelle, I am having trouble accessing the links you have posted on your website. I can not access "Quiz & Exam Schedule" "Syllabus" "Questions about the Textbook" "Welcome to Chemistry 14A" and all the "Lecture Outlines." I have tried Internet Explorer and Safari on my laptop, but nothing ever loads when I click the links. How can I resolve this?
Hi Stephanie, You need Adobe Reader which is a free download at: http://get.adobe.com/reader/
Professor Lavelle, I have a Windows computer and am having a tough time trying to view the video modules. It says to me to install a "Windows Media Player" to view video and I do press "install" but my computer still does not display the video. I am not sure what I am doing wrong. What do I do? And for the textbook, when do students need them by? Do I purchase the textbook during "zero week?"
Hi Vatche, Unusual to have Windows OS without Windows Media Player. Try searching online for Windows Media Player download. Otherwise email: VOH@CHEM.UCLA.EDU to see if they have suggestions (although they may suggest reinstalling Windows OS). You can also use the Science Learning Center computers in Young Hall, 4th floor, Dept. of Chemistry & Biochemistry. You can buy the textbook any time you want to.
Hi Professor Lavelle, I was doing the post assessment for Limiting Reactant Calculations and I was having trouble with number 9. The problem sates: How many moles of CO2(g) are produced when 1 kg of CaCO3(s) is used to neutralize an acid spill? The equation for the reaction at 1 atm and 25 degrees C is: CaCO3(s) + H2SO4(aq) > CaSO4(s) + CO2(g) + H2O(l) Would you mind walking me through the problem? Thanks, Terrin
Hi Terrin, Convert the 1 kg of CaCO3(s) to moles. Since the balanced chemical equation has 1 mole CaCO3(s) producing 1 mole CO2(g) you know how many moles of CO2(g) are produced.
Hi Professor Lavelle, I was doing the post assessment for Limiting Reactant Calculations and I was having trouble with number 11, which is: According to the following equation, 0.750 g of C6H9Cl3 is mixed with 1.000 kg of AgNO3 in a flask of water. A white solid, AgCl, completely precipitates out. What is the mass of AgCl produced? C6H9Cl3 + 3AgNO3 > AgCl + C6H9(NO3)3. Molar Mass: C6H9Cl3 (187.50 g/mol), 3AgNO3 (169.88 g/mol), AgCl (143.32 g/mol) I don't know what to do after I convert to grams to mols. Can I get a walkthrough of the rest of the problem? Thanks.
Hi James, First make sure chemical equation is balanced: C6H9Cl3 + 3AgNO3 > 3AgCl + C6H9(NO3)3 Then convert the 0.750 g of C6H9Cl3 to moles. Then convert the 1.000 kg of AgNO3 to moles. Then, based on the 1:3 molar ratio in the balanced chemical equation, determine which reactant is the limiting reactant and use its moles to determine how many moles of AgCl can be produced. Then convert the moles of AgCl produced to mass of AgCl produced.
Hi Professor Lavelle. I just finished the online assessments and I want to retry some of the problems. But I can't go back to the problems after submitting the assessment. When I click on the link it leads me directly to 'thank you for taking this survey' page.
Hi Hojin, Close and restart your browser. You should be able to retake them as many times as wanted by clicking on the link on my class web site.
Hi Professor Lavelle, I was doing the Balancing Chemical Equations assessment and when I came across question "8. During a summer camping weekend 4 moles of butane (C4H10) gas were used for cooking. Chose the right balanced equation for the combustion of 4 moles of butane gas. What is the net number of moles of gas produced?" I did not understand how to come up with the product side of the chemical equation. I thought that side was a given, or maybe I just can't see it. So if you can please answer question 8 or explain how I can come up with the product side I would really appreciate it!
Hi Cindy, Combustion of a hydrocarbon always results in the products CO2 and H20.
Hi Professor Lavelle. I just finished the online assignments and i was wondering if you could go over some materials in class such as photoelectric effect and atomic spectra, and how to use the formulas. Thank you!
Hi Minh, In class I will go through those topics in detail. You are welcome.
Hello Professor Lavelle. How do we know how many decimal places to use during our calculations of a problem? I understand that we're supposed to round for significant figures for the final answer, but what about for those steps leading up to the answer? Do we round to significant figures while calculating as well? Will we be marked down for using too many decimal places and waiting until the end to round and incorporate significant figures? Should every answer on homework and test problems be rounded to significant figures?
Hi Teresa, Only round off at the very end. See course reader for examples. Yes all calculations should have appropriate significant figures.
Hi Professor Lavelle. I watched the lecture on the Photoelectric Effect about three times now, but I still do not know how to use the equations correctly. I am having trouble trying to solve this problem in the assessment. 16. Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 105 m.s1. The work function for sodium is 150.6 kJ.mol1. Answer the following three questions. A. What is the kinetic energy of the ejected electron? B. How much energy is required to remove an electron from one sodium atom? C. What is the frequency of the incident light on the sodium metal surface? If you can, could you give me a step by step explanation on how to solve this? Once school starts, I plan to go to you for a clearer understanding on the topic. Thanks so much!  Micah
Hi Micah, We will cover this in detail in class. Doing the Photoelectric Effect now is well ahead of schedule. For now the most important modules to review are: Empirical and Molecular Formulas; Balancing Chemical Equations; Limiting Reactant Calculations; and Molarity and Dilution of a Solution. The steps to solve the problem are: 1. What is the kinetic energy of the ejected electron? Use Ek = 1/2 mv^2 2. How much energy is required to remove an electron from one sodium atom? Divide the total energy (work function) by Avogadro's number. 3. What is the frequency of the incident light? Use: kinetic energy of e = energy of incoming photon – work function ***** In addition to covering this in class, the course reader has detailed examples as does the textbook.
Do we need the newest version of the Chemistry book? Would it be fine if students just bought the 4th edition instead or is it recommended to get the 5th edition.
Hi Albert, Please see link titled: Questions About The Textbook?
In your lecture, you said, "photon has no rest mass, but it does have momentum and energy." Momentum equals to mass x velocity. Since photon has no rest mass, how do you calculate its momentum? Thank you. Alice Wang
Hi Alice, Electromagnetic radiation (i.e., light) definitely has energy (or energymomentum). A hypothetical space ship could be imagined to have large sales with photons of light hitting the (large) sails (like wind against regular boat sails) with the photon's energymomentum propelling the space ship forwards. The classical mechanics equation p = mv applies to objects that have 'rest mass' meaning everyday type objects. Light cannot have rest mass because it ceases to exist if it is at rest (i.e., light is something that moves, more specifically at 299,792,458 m/s in a vacuum). Equations that apply to classical objects (i.e., they have rest mass) don't apply to electromagnetic radiation (i.e., light). For light, E = h x frequency = h x c/wavelength. Since wavelength = h/p we can relate wavelength and momentum (p) and one can calculate p knowing the (experimentally determined) wavelength of the light. Now you see the importance of the De Broglie equation. The short answer to your question is the equation p = mv does not apply to light but one can calculate the momentum of light using the De Broglie equation. One can also calculate the energy of light using E = h x frequency or E = h x c/wavelength.