Deck 8: Vacuum Distillation

Question 1

Predict the boiling points of the following compounds at the pressure indicated.
(a) water at 745 mm Hg
(b) dichloromethane (bp 40˚ at 760 mm) at 737 mm
(c) benzene (bp 80˚ at 760 mm) at 765 mm

Answer 1

(a) bp = 100˚ - [(0.5˚ / (10 mm x 15 mm)] = 99.25˚, or about 99˚
(b) 39.85, or about 40˚
(c) 80.25˚, or about 80˚

Question 2

Using the nomograph in Figure 7.6, approximate the boiling points of the following sub- stances at atmospheric pressure (760 mm Hg).
(a) rotenone, bp 210˚ (0.5 mm Hg)
(b) 1,6-hexanediol, bp 132˚ (9 mm Hg)

Answer 2

(a) about 500˚ at 760 mm Hg
(b) about 270˚ at 760 mm Hg

Question 3

Approximate the boiling points of the compounds in problem 7.2 at 25 mm Hg.

Answer 3

(a) 340˚ at 25 mm Hg
(b) 170˚ at 25 mm Hg

Question 4

At low pressures such as used in vacuum distillation, reducing the pressure by half reduces a boiling point by about 10˚. If a compound boils at 180˚ at 10 mm Hg, what would be its approximate boiling point at the following pressures?
(a) 15 mm Hg
(b) 2.0 mm Hg

Question 5

Explain why a vacuum distillation apparatus should be checked for leaks before the material to be distilled is placed in the flask.

Question 6

Explain how you would purify octyl acetate from a mixture of octyl acetate and methylene chloride. (You will first need to look up the boiling point of octyl acetate in a suitable hand- book, Internet site, or online source. See Technique 17, p. 198.)