How do you calculate the vapor pressure of a solution?

1 Answer
Stefan V.
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

Psolution=χsolvent⋅P0solvent, where

Psolution - the vapor pressure of the solution;
χsolvent - the mole fraction of the solvent;
P0solvent - 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

Psolution=χsolvent⋅P0solvent+χsolute⋅P0solute, where

P0solute - the vapor pressure of the pure solute;
χsolute = the mole fraction of the solute.

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

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