For a solution containing volatile solute:
In equilibrium the vapour consist of both the components of the solution. The component having more vapour pressure will have more mole fraction in vapour phase.
Raoult's Law:
Partial vapour pressure of each component depends on the mole fraction of the corresponding component.
Partial pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture.
For a component 1:
p=p∘1χ1
Here p is the partial pressure of component 1.
p∘1 is the vapour pressure of the component 1.
χ1 is the mole fraction of component 1 in the mixture.