A liquid mixture of 'A' and 'B' (assume ideal solution) is placed in a cylinder and piston arrangeme — Solutions and Colligative Properties Chemistry Question

Question

A liquid mixture of 'A' and 'B' (assume ideal solution) is placed in a cylinder and piston arrangement. The piston is slowly pulled out isothermally. At the instant when the quantity of the liquid still remaining is negligibly small, the mole fraction of 'A' in the vapour is 0.4. If P_A° = 0.4 atm, P_B° = 1.2 atm at this temperature, the total pressure at which the liquid has almost evaporated, is