Deck 9: Steam Distillation

Question 1

A mixture of immiscible liquids (both water-insoluble) is subjected to steam distillation. At 90˚, the vapor pressure of pure water is 526 mm Hg. If the vapor pressure of compound A is 127 mm Hg and that of B is 246 mm Hg at 90˚:
(a) what is the total vapor pressure of the mixture at 90˚?
(b) would this mixture boil at a temperature above or below 90˚?
(c) what would be the effect on the vapor pressure and boiling temperature by doubling the amount of water used?

Answer 1

(a) 899 mm Hg
(b) below 90˚
(c) no effect

Question 2

Suggest a reason why benzene steam-distills at the rate of1 g / 0.1 g H₂O, but nitrobenzene dis- tills at the rate of 1 g / 4 g H₂O.

Answer 2

Benzene is more volatile (bp 80?) than is nitrobenzene (bp 210?).

Question 3

At 99˚, the vapor pressure of water is 733 mm Hg.
(a) At standard atmospheric pressure, what is the vapor pressure of a compound being steam-distilled at this temperature?
(b) If the compound has a molecular weight of 180, how much water is required to distill 1.0 g of the compound?

Answer 3

(a) P_total = P˚ _A + P˚ _B
If P˚ _A (water) is 733 mm and P_total is 760, then P˚ _B (the compound being distilled) is (760 - 733) or 27 mm.
(b) (wt_A / wt_B) = (P˚ _A / P˚ _B) x (MW_A / MW_B)
wt_A = 1.0 x (733 / 27) x (18 / 180) = 2.7 g of water

Question 4

At 99.6˚, water has a vapor pressure of 750 mm Hg and quinoline (immiscible with water) has a vapor pressure of 10 mm Hg. What weight of water must be distilled for each gram of quin- oline in a steam distillation at 760 mm Hg?

Question 5

Aniline (MW 93) co-distills with water at 98.2˚. The partial vapor pressure of water at 98.2˚ is 712 mm Hg. You need to isolate 10 g of aniline. What is the minimum amount of water it will take to steam-distill this amount of aniline?

Question 6

Limonene (MW 136) is a pleasant-smelling liquid found in lemon and orange peels. The bp of limonene is 175˚, but it co-distills with water at 97.5˚. If the vapor pressure of water at 97.5˚ is 690 mm Hg, what percentage of the steam-distillate is limonene?