CHEM-332 FINAL STUDY GUIDE WITH COMPLETE
SOLUTION!!
Why is it important to allow solution recrystallization to occur slowly and not let
crystals rapidly crash out of solution?
Answers :This can also allow impurities to precipitate
At the end of solution recrystallization, where are the impurities expected to be?
Answers :In the solution (if soluble) or in the filter (if insoluble)
How much solid benzoic acid is expected to be collected from recrystallization of
6g in 2L hot water upon cooling to 25oC? Benzoic acid's solubility is 3.4g/L at
25oC. Also explain why this illustrates the importance of using the minimum
amount of hot solvent for solution recrystallization purification.
Answers :0g. 6g of benzoic acid will still be soluble in 2L of water at room
temperature. Using too much solvent can cause you to lose large amounts of your
product.
What are the two indications of an impure solid when using melting point analysis?
Answers :Sample changes color, melting point is lower than expected
Why is the vapor pressure of a liquid temperature dependent?
Answers :Higher temperature increases the speed of molecules in the liquid,
allowing them to escape
Why is the boiling point of a pure liquid in an open container dependent on the
external pressure?
Answers :The vapor pressure of the liquid must be greater than the atmospheric
pressure for it to boil.
, Is the boiling point of a pure liquid higher or lower than that of the same liquid
with impurities at a given pressure?
Answers :Higher, due to Raoult's law: Px = PxoNx, where x is the substance of
interest, Px is its vapor pressure, Pxo is the vapor pressure of pure x, and Nx is the
mole fraction of x in solution.
If the liquid contains impurities, the mole fraction of x will be <1, therefore the
vapor pressure of x will be lower than its vapor pressure as a pure substance.
During simple distillation, why does the temperature need to be gradually
increased over time to maintain distillation? Use Raoult's Law to justify your
answer.
Answers :Raoult's law: Px = PxoNx
As the solution boils, x is being removed from the solution, so its mole fraction
decreases. Since Pxo is constant, Px decreases, so the boiling point of x in solution
increases.
For reflux fractional distillation of the same A + B ideal solution discussed above,
which vapor is expected to be obtained in high composition in the beginning of the
distillation?
Answers :B
Describe how a fractional reflux column works to purify a binary mixture with
volatile constituents.
Answers :As the two liquids boil together in the bottom flask, their vapor rises up
the distillation column, where it condenses. The heat from the rising vapor then re-
vaporizes the liquid. Each time the liquid is vaporized, its concentration of the
lower-boiling liquid will increase. Ideally, when it reaches the top of the column,
SOLUTION!!
Why is it important to allow solution recrystallization to occur slowly and not let
crystals rapidly crash out of solution?
Answers :This can also allow impurities to precipitate
At the end of solution recrystallization, where are the impurities expected to be?
Answers :In the solution (if soluble) or in the filter (if insoluble)
How much solid benzoic acid is expected to be collected from recrystallization of
6g in 2L hot water upon cooling to 25oC? Benzoic acid's solubility is 3.4g/L at
25oC. Also explain why this illustrates the importance of using the minimum
amount of hot solvent for solution recrystallization purification.
Answers :0g. 6g of benzoic acid will still be soluble in 2L of water at room
temperature. Using too much solvent can cause you to lose large amounts of your
product.
What are the two indications of an impure solid when using melting point analysis?
Answers :Sample changes color, melting point is lower than expected
Why is the vapor pressure of a liquid temperature dependent?
Answers :Higher temperature increases the speed of molecules in the liquid,
allowing them to escape
Why is the boiling point of a pure liquid in an open container dependent on the
external pressure?
Answers :The vapor pressure of the liquid must be greater than the atmospheric
pressure for it to boil.
, Is the boiling point of a pure liquid higher or lower than that of the same liquid
with impurities at a given pressure?
Answers :Higher, due to Raoult's law: Px = PxoNx, where x is the substance of
interest, Px is its vapor pressure, Pxo is the vapor pressure of pure x, and Nx is the
mole fraction of x in solution.
If the liquid contains impurities, the mole fraction of x will be <1, therefore the
vapor pressure of x will be lower than its vapor pressure as a pure substance.
During simple distillation, why does the temperature need to be gradually
increased over time to maintain distillation? Use Raoult's Law to justify your
answer.
Answers :Raoult's law: Px = PxoNx
As the solution boils, x is being removed from the solution, so its mole fraction
decreases. Since Pxo is constant, Px decreases, so the boiling point of x in solution
increases.
For reflux fractional distillation of the same A + B ideal solution discussed above,
which vapor is expected to be obtained in high composition in the beginning of the
distillation?
Answers :B
Describe how a fractional reflux column works to purify a binary mixture with
volatile constituents.
Answers :As the two liquids boil together in the bottom flask, their vapor rises up
the distillation column, where it condenses. The heat from the rising vapor then re-
vaporizes the liquid. Each time the liquid is vaporized, its concentration of the
lower-boiling liquid will increase. Ideally, when it reaches the top of the column,
