Hexane ( $P^{0} = 150$ torr) and heptane ( $P^{0} = 45$ torr) form an ideal solution at a given temperature with mole fraction of hexane $0.40$ in the liquid. The mole fraction of hexane in the vapour in equilibrium is

Answer: A

Partial pressures:

p_{h e x} = 0.40 \times 150 = 60.0

torr and

p_{h e p} = 0.60 \times 45 = 27.0

torr, so

P = 87.0

torr. Dalton's law gives

y_{h e x} = 60.0/87.0 = 0.690

. The vapour is richer in the more volatile component.