Chem 241 Final Exam Review With
Complete Solutions
At what pH ranges would compound A with pKa=6 and compound B with pKa=9
be 90% in the same solution layer? - ANSWER Compound A: pH 0-5 is organic
layer, pH 7-14 is aqueous. Compound B: pH 0-5 is aqueous, pH 10-14 in organic.
Compounds in the same layer at pH=7-8
How does the rate of cooling impact the purity of crystals? - ANSWER The
slower the cooling, the slower the crystals form, increasing purity.
State two changes in melting point of a compound before, and after
crystallization. - ANSWER The melting point after crystallization will be more
narrow and higher than the impure melting point, closer to the literature value.
If too much solvent was added and the target doesn't form crystals, a common
trick is to boil away some of the solvent and allow the solution to cool again. Why
are we not worried about accidentally boiling away our target? - ANSWER The
targets are solids and have a very high boiling point exceeding conditions. The
solute will not boil away.
Which method of distillation is more effective at separating mixtures, simple or
fractional? - ANSWER Fractional. The vapor condenses and re-boils multiple
times, increasing purity. Fractional distillation is like multiple simple distillations
at once.
Briefly explain why we don't distill to dryness? Make sure your answer gives a
specific reason. - ANSWER To prevent the remaining material from
overheating/polymerizing. It is also unsafe as it may compromise the integrity of
the glassware.
You obtain the following NMR data of distillate, MTBE and dimethyl carbonate.
What is the % composition of the mixture? [(shift: 1.2, 9H's), (shift: 3.2, 3H's),
(shift: 3.8, 54H's)] - ANSWER MTBE: peak at 1.2, 9H's/9H's per molecule is one
molecule/ peak at 3.2, 3H's/3H's per molecule is one molecule.
DMC: peak at 3.8, 54H's/6H's per molecule is 9 molecules.
MTBE: 10%, DMC: 90%
What would be the composition of the first or last mL of distillate? Briefly explain
your reasoning. - ANSWER First mL: Mixture is enriched with low boiling point
compound which distills faster.
Complete Solutions
At what pH ranges would compound A with pKa=6 and compound B with pKa=9
be 90% in the same solution layer? - ANSWER Compound A: pH 0-5 is organic
layer, pH 7-14 is aqueous. Compound B: pH 0-5 is aqueous, pH 10-14 in organic.
Compounds in the same layer at pH=7-8
How does the rate of cooling impact the purity of crystals? - ANSWER The
slower the cooling, the slower the crystals form, increasing purity.
State two changes in melting point of a compound before, and after
crystallization. - ANSWER The melting point after crystallization will be more
narrow and higher than the impure melting point, closer to the literature value.
If too much solvent was added and the target doesn't form crystals, a common
trick is to boil away some of the solvent and allow the solution to cool again. Why
are we not worried about accidentally boiling away our target? - ANSWER The
targets are solids and have a very high boiling point exceeding conditions. The
solute will not boil away.
Which method of distillation is more effective at separating mixtures, simple or
fractional? - ANSWER Fractional. The vapor condenses and re-boils multiple
times, increasing purity. Fractional distillation is like multiple simple distillations
at once.
Briefly explain why we don't distill to dryness? Make sure your answer gives a
specific reason. - ANSWER To prevent the remaining material from
overheating/polymerizing. It is also unsafe as it may compromise the integrity of
the glassware.
You obtain the following NMR data of distillate, MTBE and dimethyl carbonate.
What is the % composition of the mixture? [(shift: 1.2, 9H's), (shift: 3.2, 3H's),
(shift: 3.8, 54H's)] - ANSWER MTBE: peak at 1.2, 9H's/9H's per molecule is one
molecule/ peak at 3.2, 3H's/3H's per molecule is one molecule.
DMC: peak at 3.8, 54H's/6H's per molecule is 9 molecules.
MTBE: 10%, DMC: 90%
What would be the composition of the first or last mL of distillate? Briefly explain
your reasoning. - ANSWER First mL: Mixture is enriched with low boiling point
compound which distills faster.
