P°A = 7×10³ Pa, P°B = 12×10³ Pa at 350 K. Liquid contains 40 mol% A. Find vapour composition.

Step-1: Partial pressures (Raoult’s Law):

PA = 0.40 × 7000 = 2800 Pa

PB = 0.60 × 12000 = 7200 Pa

Step-2: Total pressure:

P_total = 2800 + 7200 = 10000 Pa

Step-3: Vapour mole fractions (Dalton’s Law):

yA = 2800 / 10000 = 0.28

yB = 7200 / 10000 = 0.72

Answer: Mole fraction of A in vapour = 0.28, B in vapour = 0.72

The more volatile component (B, higher P°) is enriched in the vapour. This is the principle behind fractional distillation.