Raoult's law

Raoult's law states that the vapour pressure of a binary solution containing a non-volatile solute is directly related to the mole fraction of solvent (i.e. volatile) in the solution.

Also, it states that the vapour pressure of each component in a binary solution containing volatile components is directly related to its respective mole fraction in the solution.

Relative going-down of vapour pressure is equal to mole fraction of non volatile and non-electrolytic solute. This is known as Raoult's law.

P-P(INITIAL)/P(INITIAL)=n(SOLUTE)/n(SOLVENT)

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Raoult's Law Media

Vapor pressure of a binary solution that obeys Raoult's law. The black line shows the total vapor pressure as a function of the mole fraction of component B, and the two green lines are the partial pressures of the two components.
Positive and negative deviations from Raoult's law. Maxima and minima in the curves (if present) correspond to azeotropes or constant boiling mixtures.