It is a law about thermodynamics. It is the vapor pressure of a solution of a non-volatile solute is equal to the vapor pressure of the pure solvent at that temperature multiplied by its mole fraction. More simplified the vapour pressure, of a binary solution, containing a non-volatile solute, is directly related to the mole fraction of solvent in the solution. It also says that vapour pressure of each component in a binary solution containing volatile components is directly related to its respective mole fraction in the solution.
