Chapter 12, Problem 12.64QE

Interpretation Introduction

Interpretation:

The vapor pressure (in torr) of cyclohexane above the solution has to be calculated.

Concept Introduction:

Vapor pressure depression:  The equilibrium between a liquid and its vapor produces a characteristic vapor pressure for each substance that depends on the temperature.  The lowering of the vapor pressure is caused by a lesser ability of the solvent to evaporate, so equilibrium is reached with a smaller concentration of the solvent in the gas phase.  The vapor pressure of a solution is expressed using Raoult’s law:

  ${\text{P}}_{\text{solv}}{\text{=χ}}_{\text{solv}}{\text{P}}^{\text{o}}{}_{\text{solv}}$

The vapor pressure of the solvent $\left({\text{P}}_{\text{solv}}\right)$ above a dilute solution is equal to the mole fraction of the solvent ${\text{(χ}}_{\text{solv}}\text{)}$ times the vapor pressure of the pure solvent $\left({\text{P}}^{\text{o}}{}_{\text{solv}}\right).$  The difference between the vapor pressure of the pure solvent and the vapor pressure of the solution $\text{(ΔP)}$ is proportional to the concentration of the solute and is given by the equation,

  ${\text{ΔP=χ}}_{\text{solute}}{\text{P}}^{\text{o}}{}_{\text{solv}}$

The vapor pressure depression of the solvent is a colligative property, because it is proportional to the concentration of the solute.  Raoult’s law is applicable to dilute solution.