The vapour pressure of benzene at a certain temp. is 640 mm Hg . A non-volatile-non-electrolyte solid weighing 2.175g is added to 39.0 g of benzene. The vapour pressure of the solution is 600 mm Hg. What is the molecular weight of the solid substance? SOLUTIONS

0 640 Benzene P mm of Hg (vapour pressure of pure solvent) 600 Benzene P mm of Hg (vapour pressure of solution) 2.175 B W g (wt of solute) ? B M (m. wt of solute) 0 0 39 78 / 640 600 78 2.175 640 39 78 2.175 640 40 39 69.6 / Benzene Benzene Benzene Benzene Benzene Benzene B B Benzene B B B W g M g mole P P M W M W P M M M g mole x x x x x x x Thus, the correct option is (d).