# Raoult's law

Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present: Raoult's Law is a law that relates the the vapor pressure of a solution is dependent on the mole fraction of a solute added to solution.

Psolution = ΧsolventP0solvent

where

Psolution is the vapor pressure of the solution

Χsolvent is mole fraction of the solvent

P0solvent is the vapor pressure of the pure solvent

### Limitations on Raoult's Law

Ideal solutions satisfy Raoult's Law. The solution in the last diagram of Figure 3 above would not actually obey Raoult's Law - it is far too concentrated, but was drawn so concentrated to emphasized the point.

In an ideal solution, it takes exactly the same amount of energy for a solvent molecule to break away from the surface of the solution as it did in the pure solvent. The forces of attraction between solvent and solute are exactly the same as between the original solvent molecules - not a very likely event!