Liquids A and B form an ideal solution. At a given temperature $P_{A}^{0} = 300$ torr and $P_{B}^{0} = 100$ torr. The vapour in equilibrium with a solution has mole fraction of A equal to $0.60$ . Find the total vapour pressure (torr) of the solution, to the nearest integer.

Numerical answer type, answer in torr, tolerance 1.

Answer: 167 torr

Let liquid mole fraction be

x

y_{A} = 300 x / [300 x + 100 (1 - x)] = 3 x / (1 + 2 x) = 0.60

, so

3 x = 0.60 + 1.2 x

and

x = 1/3

P = 300 (1/3) + 100 (2/3) = 100 + 66.67 = 166.67

torr, i.e. about 167 torr.