A solution contains 50.0 g of heptane (C₇H₁₆) and 50.0 g of octane (C₈H₁₈) at 25 °C. The vapor pressures of pure heptane and pure octane at 25 °C are 45.8 torr and 10.9 torr, respectively. Assuming ideal behavior, answer the following: d. Why is the composition of the vapor different from the composition of the solution?

Key Concepts

Raoult's Law

Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent in the solution. This principle helps explain how the presence of different components in a solution affects the overall vapor pressure, leading to variations in the composition of the vapor compared to the liquid phase.

Vapor-Liquid Equilibrium

Vapor-liquid equilibrium refers to the state where the rate of evaporation of a liquid equals the rate of condensation of its vapor. In a mixture of volatile liquids like heptane and octane, the equilibrium composition of the vapor will differ from that of the liquid due to differences in their individual vapor pressures and mole fractions.

Ideal Solutions

An ideal solution is one where the interactions between different molecules are similar to those between like molecules, leading to predictable behavior according to Raoult's Law. In the case of heptane and octane, assuming ideal behavior allows us to use their vapor pressures to determine how their mixture will behave, including the differences in vapor composition.