The change in the vapor pressure due to the presence of a solute is governed by Raoult's law. The presence of a solute decreases the vapor pressure of the pure solvent. Since vapor pressure is a colligative property, the lowering of the vapor pressure must depend on the amount of solute present. The amount of solute present is represented in Raoult's law by the mole fraction of the solute. The new vapor pressure of the solvent, designated by is given by
where is the mole fraction of the pure solvent in the solution and is the original vapor pressure of the pure solvent. The vapor pressure of the solute can be found in a similar way:
