# How do you calculate the vapor pressure of a solution?

Dec 24, 2014

This is done by using Raoult's law, which states that, for an ideal solution, the partial vapor pressure of a component in that solution is equal to the mole fraction of that component multiplied by its vapor pressure when pure.

If you're dealing with a solution that has a non-volatile solute (a solute that does not have the tendency to form vapor at the temperature of the solution), then the vapor pressure of that solution is

${P}_{s o l u t i o n} = {\chi}_{s o l v e n t} \cdot {P}_{s o l v e n t}^{0}$, where

${P}_{s o l u t i o n}$ - the vapor pressure of the solution;
${\chi}_{s o l v e n t}$ - the mole fraction of the solvent;
${P}_{s o l v e n t}^{0}$ - the vapor pressure of pure solvent.

The mole fraction simply refers to ratio between solvent moles and the total number of moles in the solution.

If however you're dealing with a solution that contains a volatile solute, the vapor pressure of that solution is

${P}_{s o l u t i o n} = {\chi}_{s o l v e n t} \cdot {P}_{s o l v e n t}^{0} + {\chi}_{s o l u t e} \cdot {P}_{s o l u t e}^{0}$, where

${P}_{s o l u t e}^{0}$ - the vapor pressure of the pure solute;
${\chi}_{s o l u t e}$ = the mole fraction of the solute.

This equation can be extended to solutions containing three or more components.