Raoult’s law states that the relative lowering of vapor pressure of a solution to pure solvent containing a non-volatile solute is equal to the mole fraction of the solute in the solution.
The vapor pressure of a solution Ps is less than that of pure solvent P0. Thus, lowering in vapor pressure of solution ΔP = P0 – Ps
Or (P0-Ps)/P0 = Xn1 = n/N
(P0-Ps)/P0 = wA/mA X mB/wB
n – moles of solute, N – moles of solvent, w – weight of solute, m – molar mass of the solute, W – weight of solvent, M – molar mass of solvent, Xn1 – mole fraction of solute