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| Q&A
for 14BL |
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For buffer solutions, if the concentration of the acid is equal to that of its conjugate base, is the pH of the solution equal to the pka regardless of the ka value, or must the ka be less than 10^-5?
Only for Ka less than or equal to 10^(-5).
Dr. Pang, for the kinetics lab, why wasn't any bleach used (in addition to H20) to prepare the blank solution?
Actually, the bleach can be included as part of the blank but in that case, one will have to prepare a different blank for each of the trial. The bleach concentrations are not the same for all trials and therefore, one will have to consider that as well.
Hey Professor Pang, where are the solutions posted for the midterm?
On the first floor of Young Hall (across from the vending machines).
Dr. Pang, Are the units for ln([dye]) mol/L, or is this value unitless? Thanks.
The unit is still mol/L.
hi prof. i am confused about what you are asking for in experiment 5's post lab, involving the initial concentration of sol'n 3 for all 6 trials. do we just find the concentration after we have added the water?
Concentration is always refers to as the amount of solutes in a solution with a certain TOTAL volume. The total volume for each of the trial is the same.
Hi Professor Pang. The Mohrig text says that a solid mixture of A+B will first begin melting at the eutectic temperature. However, in the phase diagram on your lecture handout, it shows that the first drop of liquid appears at a temperature above the eutectic temperature. Can you please clarify?
This is correct. The phase diagram that I gave all of you in lecture is not the same as the binary micture heating curve. If you look at the binary mixture heating curve (p.94, Figure 10.3), you will see that the initial "melting" actually starts at the eutectic temperature and then slowly reaching the experimental melting point accroding to the heating curve.
In calculating the error analysis for aspirin, do we include the error of the balance and if so, what is this error.
Refer to post-lab report for experiment #2 and 3a with regard to error analysis. You have done this type of error propagation in those two experiments. One of them require you to use the balance as one of the sources of error.
Dr. Pang, I am having accessing the website that we need for this CPR. Who should I email about this problem? Thanks.
I checked the Web link myself and it is working fine. If you have trouble linking to the Web site, email me directly.
Hi Professor. I was wondering if it would be possible to request a regrade for the CPR? Thanks.
Please refer to the previous comments
Hi Prof. Pang, Are there regrades on the CPR?
Any concerns about CPR grading should be addressed to me via email.
Hello Professor, I wanted to know for the one full titration graph needed for the post lab of part one do we just choose one or do we put all the data sets from all the three trials into one graph. Thanks
Choose the best trial for the full titration graph.
Thank you. My next question involves #4C regarding the absolute error in molarity for Mg(OH)2; your answer in the key is rounded up to .0001 whereas the final answer in my calculator is 8.37^10-5...most of the other answers so far have not been rounded in this fashion - what to do?
You need to make sure that the absolute error agrees with the quantity in terms of the numbers of decimal places.
Hi Dr. Pang Are you going to be providing us with the inherent error for any equipment in the questions or should we go by the assumption that it is +/- 1 (to the last digit)?
Absolute errors for various glassware / equipment will be provided on the exam if they are needed.
Hey Prof. Pang, For problem 9a on the midterm practice problems: I am having trouble getting the concentration of H+ to match your answer. I even used the quadratic forumula to solve for the moles of H+ because Y wasn't sufficiently small. The equation I had was 1.40x10^-3 = (x^2)/(5x10^-4 - x) and I got x (the concentration of H+) to equal 0.00782. This resulted in a pH of 2.1. How did I go wrong?
Your mistake is that you used substitute the value of mole instead of molarity into the equilibrium constant. 5X10^(-4) is the amount of acid in mole. The concentration is 0.0100M. I normally use moles in ICE box but then converted back to molarity before substituting them back to the equilibrium expression. Refer to the lecture notes for more details.
On the practice test, when you are asked to write the formula of the conjugate bases of molecules, the reaction is NH4Cl+OH ---> NH3Cl+H2O, but why is the conjugate base only NH3 not including the Cl?
The NET ionic equation can simply be written as, NH4(+) + OH(-) = NH3 + H2O The weak base is NH3 and its conjugate acid is NH4(+).
Hi Professor Pang, FOr the post-lab (vinegar Titration), should we have our graphs on seperate sheets from the data like the excel workshop handout says or should we place the graphs on the same sheet as the corresponding data tables like in the sample data sheet you gave us?
Separate the graphs and the data tables. Don't put them on one sheet. I did that for the lecture note because I want save paper when photocopying the notes for the class.
I have a question regarding the prelab for part b of lab 3. for the study questions. It says that week 1 should be #3 and week 2 should be #2 and 3, but in the prelab guidelines for week one, you said to do #1. would you please clarify? Thanks.
There are TWO pre-lab reports for part 1 & part 2. Refer to the lab schedule for the due date of each pre-lab report.
Hi Dr. Pang, I am wondering if there is a problem with the CPR website because I can't get to the site from the VOH site. Thanks!
Please refer to the previous comment on VOH.
The link to the CPR site isn't working for me and some other classmates. I've tried going to the site directly by typing in the address, and that doesn't seem to work either. When I use Internet Explorer, it says that the page can't be found. When I use firefox, it says that the connection to the site was refused. Is the site down? If not, who should I talk to?
The CPR program was transferred to a new server over the weekend. The program is now working.
For post lab 2, we are asked to find the absorbances of the solutions at 550nm. Don't we need the molar extinction coefficient for 550nm (we are given the coefficient for 620nm)
No, you don't need to know the molar extinction coefficient at 550nm. Think about Beer's law and also refer to the online Beer's law tutorials. The online tutorials should be quite useful for the report.
Hi Professor Pang, I was able to log into CPR this morning but I've tried all night and have not been able to connect to the page. Can you please check and see if the site is working properly? Thanks a lot.
The CPR server is now up and running. The program was being transferred to a new server over the weekend.
Hey Prof. Pang, I just wanted to make sure, because my TA didn't mention it in lab; we turn in one postlab report per pair of lab partners for lab 2, right?
Please refer to the guidelines posted on VOH. At the beginning of the post-lab report guideline, I always specified whether the report is a GROUP report or INDIVIDUAL report.
I am unable to reach the chemfinder website for information on the blue food coloring for the second lab. It claims the sight does not exist. How do I get the information needed? Thanks
I just checked and the Chemfinder site was working.
Hi Dr. Pang, can you explain to me what "absorbances" are again, and how they are related to concentrations? Thanks.
Please refer to lecture guide and the online tutorial.
Dear Professor Pang, I am having trouble accessing the "8(e) TRIAGE Chemical Studies Database" link from the CS ChemFiner Web Site since it requires installation and there are restrictions on the computer lab accounts. Suggestions? Thank you Shireen
It's odd. I can use the CS Chemfinder on my PC without any installations. It shouldn't need any installations since it is simply a Web search engine.
hi Professor! when are the cpr's due?
I will provide information with regard to the first CPR writing assignment next week (10/20) in lecture.
Hi Professor- I was wondering if we needed to answer the questions under Application and Practical Considerations for the post-lab report. These are located in our lab manual on page 38.
Refer to the guideline posted on VOH.
Hi everyone! I have a conflict with my lab section. It's on Thursdays (1H) from 2-5. Please contact me if you're interested! Thanks!!
If any one of you can switch the lab with this student, please email the student as soon as possible. Make sure that both of you come and see me for proper enrollment changes.
Hi, I was wondering if anyone would like to switch lab sessions? I have mine on Thursday from 2-5 and was wondering if anyone would like to switch. Thanks Wilson
Please email the student if you can switch lab with him. Make sure that both of you will need to come and see me for enrollment changes.
Hi Dr. Pang. I was just wondering if the method for processing data or recording it needed to be included in the flow chart or if the flow chart only needed to have the procedures in it. Also, can we make tables to record data in in our lab books before coming to lab? Thank you.
You can simply write what need to be recorded and whether any RAD or RSD analysis is needed to check for appropriate precision.
Professor, Is this an acceptable format for the reference part of our prelab? Books without an Editor: Calvert, J.G.; Pitts, J.N. Photochemistry; Wiley: New York, 1966; pp 156-186.
Refer to the ACS citing style for appropriate reference style. You can find the ACS citing style from the course Web site. There is a link on 14BL VOH that will take you directly to the course Web site. Click on "Course Resources". One of the resources on that page is for citing references.
Hello Professor, For the Pipet Pre-lab are we supposed to do the Prelab Assignment and include it in the pre-lab? If so should the answers be located last in our pre-lab after the flow chart?
I assume you are referring to the study questions. Study questions should be the section after the procedural flowchart in your pre-lab report.
Hi Dr. Pang, I was wondering for the procedures, do we have to put it in a flowchart form or can we just number it? Thanks.
The procedures should be constructed in flowchart format.
Hi Dr. Pang, I tried to go to the link you gave me: http://pubs.acs.org/books/references.html, but the site said "Page not found". What else can I do? I tried looking in the course website under course resources, but could not find anything related.
I just checked the linked it works. You can simply go to the course Web site. There is a link on 14BL VOH that takes you there. Click on "Courses Resources" and you will see there are lots of useful links on that page. One of them is for scientific writing. Here is the link again. http://pubs.acs.org/books/references.shtml J. Pang
Hey professor, I tried clicking on the lab video link but the page that came up said that the item that was searching for could not be found. Any thoughts as to what I can do?
If you have trouble using the VOH link, simply go to the OID Web link that I listed on the guideline. Click on the corresponding video title on OID Web site to download the video. Make sure you have Real Player installed on your computer.
I am in lab 1H on Thursdays 2-5. It meets in WGYoung 1379. Unfortunately I have a schedule conflict. I'm searching for anyone willing to trade days with me because the class is impacted. Please email me at Caspiras@ucla.edu or i/m me at "bitbyajunbug." Thanks!
If anyone of you can make the switch, please email this student as soon as possible.
Dr. Pang, for the pre-lab that is due next Thursday, do you want us to use MLA or APA as our guideline for the Reference section? More specifically, how do you want us to do it and can you give us an example? Thank you very much for your help.
In chemistry, we use the ACS (American Chemical Society) citing style when referencing materials. Here is the link. http://pubs.acs.org/books/references.shtml You can also locate the same link and many other information under "Course Resources" in our course Web site. There is a link on VOH that takes you directly to the course Web site. J. Pang
