1. An organic chemistry lab book gives the following solubility data for oxalic acid
|9.5 g/100mL of water||23.7 g/100 mL of ethanol||16.9 g/100 mL of ether|
Answer: Since ethanol and water are miscible, you must use ether.
(b) Calculate the partition coefficient for your choice. (Be sure to define the terms of your equation.)
Answer: Defining the partition constant Kp, as solubility in ether/solubility in water gives Kp = 16.9/9.5 = 1.78
(c) Calculate the weight of acid remaining in the water phase if you extracted the 40 g/1000 mL of water with 1000 mL of the solvent you chose in part (a).
Answer: Kp = 1.78 = x/(40-x) and x = 25.6 g
(Since the volumes are the same, the ratio of the weights are the same as the ratio of the (wt/vol)ís
(d) Draw the structure of oxalic acid.
Answer: Please see lecture 2 notes.
(e) How could you extract oxalic acid from an organic phase into an aqueous phase? (Note this is a different question than was originally posted.)
Answer: You need to make the acid more soluble in the aqueous phase and less soluble in the organic phase. That is you need to increase the "ionic character" of the acid. Adding a base to the solution will cause the necessary reaction to leave you with the oxalate ion.
At high temperature, there is too much motion
in the moleceules to mantain these hydrogen bonds and when they break the
solubility of the benzoic acid decreases.
Answer: Ethanol. Alcohols form stronger hydrogen
bonds than amines do.
(b) acetic acid ethyl ester and propanoic acid (both C4H8O2)
Answer: Propioic acid. Hydrogen bonds are possible
here also, but not in the ester.
(c) benzene (C6H6) and cyclohexane (C6H12)
Answer: Benzene. The polarizable pi bonds of the aromatic ring provide stronger van der Waals forces between the molecules than are possible in cyclohexane where sigma bonds exist.
(d) ammonium chloride (NH4Cl) and methyl chloride (CH3Cl)
Answer: Ammonium chloride. Methyl chloride is a covalent molecule. The only intermolecular forces are van der Waals attractions. Ammonium chloride is an ionic salt composed of amonium ions and chloride ions.
Answer: If the rate of temperture change is faster than the rate of transfer of heat across the capillary tube, then the thermometer, which is measuring the temperature of the environment, not the sample, will register more change than actually occurs in the sample. The result will be a larger temperature range than should be obtained, and the conclusion could be drawn that the compound is more impure that it really is.
(b) the sample is a different compound
Answer: Again the same phenomenon will occur as above. It is possible that the disapperance of the last solid will occur when the temperature of the environment is higher than the temperature of the sample in the tube. Since the impurities depress the melting point, then a final melting point above the expected one, would imply a different compound.
Answer: First balance the masses of the chromium (the coefficient is 2)
Then balance the mass of hydrogen (14 protons)
Then balance for electrical neutrality (6 electrons on the left need to be added)
Check everything is balanced.
Cr2O72- + 14H+ + 6e- = 7H2O + 2Cr3+
(b) Write a balanced redox equation for the reaction of dichromte with the oxalate ion.
Answer: The half reaction for oxalate is C2O42- = 2CO2 + 2e-
Since the reaction has 2 electrons and the dichromate in part a has 6, the oxalate reaction must be multiplied by 3 so that the sum of the two has both electrical and mass balance.
(c) Ni(NH3)5(H2O) 3Cl2 (d) Ni(NH3)4(H2O) 2Cl2
3Cl2 (f) Ni(H2O)6Cl2
Answer: There will be a braod OH stretch at the left of the characteristic region in the acid spectrum that will not be present in the acetone spectrum.
(b) ethanol and ethyl amine
Answer: This is difficult without tables. You will see two absorptions due to the asymmetric and symmetric stretches due to the -NH2 group,whereas there will ony be one in the alcohol spectrum. Hydrogen bonding will tend to broaden the bands in both spectra unless the solutions are very dilute.
(c) acetone and acetic acid methyl ester
Answer: Again you will need tables. the position of the C=O stretch is characteristic of the type of functional group it is in. In ketones, the absorptions occurs between 1725 and 1705 cm-1, in esters it is between 1765 and 1720 cm-1.
(a) What volume of alcohol would you use to recrystallize 5 g of cholesterol?
Answer: You want to dissolve the cholesterol in the minimum volume of hot solution. If 28 g dissolve in 100 g of ehtanol they you willr equire 17.9 g of ethanol to dissolve 5 g of cholesterol. The density of ethanol is 0.79 g/mL. You will need 22.6 mL of ethanol.
(b) What is the maximum weight of product you could recover in a recrystallization?
Answer: At 20 degree C, the solubility is 1.29 g/100g. The difference in solubility between 80 degree and 20 degree is 26.7 g. The percent recovery is therefore, 26.7/28 = 95.4%. In the situation in part (a) you could recover 95.4% of 5 g = 4.7 g.
C5H5N + H+ = C5H5NH+
Cobalt forms several pyridine complexes, which have the general formula Co(C5H5N)x(H2O)y2+. The specific complex that forms depends on both the concentration of the pyridine in the solution and the temperature at which the reaction is carried out.
When a complex is made the amount of pyridine in the complex can be determined through an acid-base titration. To ensure that the reaction is quantitative, a sample of the salt is dissolved in a known volume of standard acid and the excess acid is titrated with standard base. The question below deals with an analysis such as this.
Calculate the equivalent weight of the salt.
Answer: There are two sources of base (the NaOH and the ammonia in the comound) and one source of acid (the HCl.)
equivalents of acid = equivalents of base.
0.03000L x 0.1042mol/L = (0.0985mol/L x 0.00856L) + 0.2671g /(EW)
EW = 117g
(b) Which of the following choices is the most plausible formula for the compound analyzed in part (a)? Explain your reasoning. (If you do not have an answer for part (a) assume an equivalent weight of 115 g)
Answer: EW = MW/6 = 604/6 = 101
Answer: EW = MW/5 = 543/5 = 109
Answer: Eight ligands will not fit around the cobalt atom - not a plausible compound.
Answer: EW = MW/4 = 482/4 = 121 - best choice; closest to experimental value
Answer: This is an ion, not a neutral comound - not possible.
(c) If the limiting reagent in the preparation of the complex salt was 15.00 mL of a 1.00 M CoCl2 solution, and 6.20 g was recovered from the reaction, what was the percent yield in the synthesis?
Answer: Maximum yield =0.15 mol X 121 g/mol = 18.15 g
% yield = 6.20/18.15 = 34.2%
(b) The Merck Index gives the following solubility
data for the solubility for 1 g of aspirin or salicylic acid.
|Aspirin (acetylsalicylic acid)||Salicylic acid|
|300 mL of H2O||460 mL of H2O|
|12 mL diethyl ether||3 mL of diethyl ether|
Answer: solubility of aspirin in water = 0.33 g/100 mL
solubility of aspirin in ether = 8.33 g/100 mL
solubility of salicylic acid in ether = 33.3 g/100 mL
aspirin partition coefficient = 8.33/.33 = 25
salicylic acid partition coefficient = 151
(c) Assume your crude product from the aspirin synthesis contained equal weights of aspirin and salicylic acid. If you dissolved 2 g of crude product in 100 mL of water and extracted with 100 mL of ether, how much purification would you achieve? That is, what would be the ratio of weights of the aspirin and salicylic acid in the resulting ether layer?
Answer: The extraction will transfer 0.9934 g of the salicylic acid to the ether layer and 0.9215 g of the aspirin to the ether layer. The ratio of weights will be 1.08/1 or a purification of only 8%.
(d) Would extraction or recrystalliztion be a better purification technique if the crude product contained 95% aspirin and 5% salicylic acid? Explain your answer.
Answer: Because the difference in solubilities is much greater than the difference in partitioning, recrystallization is the better choice for purification.