Practice Problem 10

Assume that a mixture that contains equal amounts of pentane (C₅H₁₂) and octane (C₈H₁₈) is distilled. Describe the difference between the composition of the liquid in the distillation flask and the vapor given off when this liquid starts to boil.

Answer

Pentane (BP = 36.1^oC) is much more volatile than octane (BP = 125.7^oC). Pentane therefore begins to distill from this mixture long before octane, and the partial pressure of the pentane that collects above the solution will be much larger than the partial pressure of the octane. Although the liquid contains equal amounts of the two hydrocarbons, the vapor is much richer in pentane.