Two liquids A and B form an ideal solution. The vapour pressures of pure A and pure B at $300 K$ are $150 torr$ and $60 torr$ . For a liquid with mole fraction $x_{A} = 0.30$ , find the mole fraction of A in the vapour in equilibrium with it. Give the answer to two decimal places.

Numerical answer type, tolerance 0.005.

Answer: 0.52

By Dalton's law,

y_{A} = p_{A} / P

with

p_{A} = x_{A} p_{A}^{0}

. Here

p_{A} = 0.30 \times 150 = 45 torr

and

P = 45 + 0.70 \times 60 = 87 torr

. So

y_{A} = 45/87 = 0.5172 \approx 0.52