Question

How do you calculate the vapor pressure of a solution?

Answer 1

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_(solution) = chi_(solvent) * P_(solvent)^0`, where

`P_(solution)` - the vapor pressure of the solution;
`chi_(solvent)` - the mole fraction of the solvent;
`P_(solvent)^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_(solution) = chi_(solvent) * P_(solvent)^0 + chi_(solute) * P_(solute)^0`, where

`P_(solute)^0` - the vapor pressure of the pure solute;
`chi_(solute)` = the mole fraction of the solute.

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