According to Rault's law, for any solution the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction.
When the solute is non-volatile and non-electrolyte, only the solvent moecules are present in vapour phase and contribute to vapour pressure. Let p1 be the vapour pressure of the solvent x1 be its mole fraction p10 be its vapour pressure in the pure state. Then according to raoult's law, P1 ∝ x1 and p1 ∝ x1p10
