Deck 6: Simple Distillation

Question 1

A mixture of two miscible liquids with widely different boiling points is distilled. The temper- ature of the distilling liquid is observed to plateau and then drop before rising again. Explain the temperature drop.

Answer 1

The temperature drops because vapors of the lower-boiling comound are depleted before vapors of the higher-boiling comound can fill the distillation head.

Question 2

What will be the result of the following errors in a distillation process?
(a) The thermometer bulb is placed in the boiling liquid.
(b) The distillation flask is not securely attached to the distillation head, leaving an opening between the two pieces of glassware.
(c) Boiling chips are not added to the distillation flask.
(d) The water to the water-jacketed condenser is not turned on.

Answer 2

(a) The temperature will be of the boiling liquid, which often boils at a higher temperature than the true boiling point. The temperature should be read just below the side-arm of the distillation head.
(b) Vapors will escape to the atmosphere instead of being condensed and collected.
(c) The boiling will bump and splash low-boiling solvent into the condenser.
(d) The condenser will not cool and condense the vapors and product will be lost.

Question 3

If a mixture is distilled rapidly, the separation of its components is poorer than if the mixture is distilled slowly. Explain.

Answer 3

The added heat and excess vapors prevent an equilibium between liquid and gas. Large quan- tities of the higher-boiling compounents will distill in the early fractions.

Question 4

From the graph in Figure 5.1, p. 78, estimate the following:
(a) the boiling point of acetone at 625 mm Hg
(b) the vapor pressure of acetone at room temperature (23˚C)
(c) the percent of the total pressure contributed by acetone above an open beaker of this com- pound at room temperature (23˚) and 760 mm

Question 5

A 50% aqueous solution of ethanol (50 mL total) is distilled and collected in 10-mL fractions.
Predict the boiling range of each fraction.

Question 6

Calculate the mole fraction of each compound in the following mixtures.
(a) 95.0 g CH₃CH₂OH and 5.0 g H₂O
(b) 10.0 g CH₃OH, 10.0 g CH₃CH₂OH, and 10.0 g CH₃CH₂CH₂OH

Question 7

Use the graph in Figure 5.5 tothe following questions.
(a) Determine the composition of the liquid solution boiling at 100?.
(b) What is the composition of the vapor being given off?

Question 8

A mixture of ideal miscible liquids C and D is distilled at 760 mm Hg pressure. At the start of the distillation, the mole percent of C in the mixture is 90.0, while that of D is 10.0. The vapor is condensed and found to contain 15.0 mole percent of C and 85.0 mole percent of D. Calculate the following.
(a) The partial vapor pressures (P) of C and D in this mixture
(b) The vapor pressures (Pº) of pure C and D

Question 9

Given the following mole fractions and vapor pressures for miscible liquids E and F, calculate the composition (in mole percent) of the vapor from a distilling ideal binary solution at 150˚ and 760 mm Hg. (The vapor pressures P_Eº and P_Fº are the values at 150˚C.)
X_E = 0.40 X_F = 0.60
P_Eº = 1710 mm Hg P_Fº = 127 mm Hg

Question 10

Given a mixture of compounds G and H. At 75˚C,
X_G = 0.50 X_H = 0.50
P_Gº = 900 mm Hg P_Hº = 400 mm Hg
(a) Which compound has the lower boiling point?
(b) What is the partial vapor pressure of G? of H ?*
(c) If the applied pressure is 760 mm, will the mixture boil at 75˚C?
(d) What is the composition of the vapor at 75˚C?