Question

If the vapour pressure of pure a and pure b at 298 Kelvin are 60 and 15 torr respectively what should be the mole fraction of a in the vapour phase at this temperature in a solution that contains 20% of a in the a + b binary mixture in the liquid phase

Answer 1

Mole fraction of a = 0.2 Mole fraction of b = 0.8 Pressure of solution = 0.2x60 + 0.8x15 = 24 torr Partial pressure of a = mole fraction of a in vapour x total pressure Mole fraction of a in vapour = 12/24 = 0.5