The relationship between molar masses of the two solvents is MX=43MY The relative lowering of vapour pressure of the two solutions is (PΔP)X=m(PΔP)Y But, the relative lowering of vapour pressure of solutions is directly proportional to the mole fraction of solute. Given 5 molal solution, means 5 moles of solute are dissolved in 1 kg ( or 1000 g ) of solvent. The number of moles of solvent =M1000 g The mole fraction of solute =1000/M5 =M×10005 hence MX×10005=m×MY×10005. Substitute equation (i) in equation (ii) 43×MY×10005=m×MY×10005 m=43