Liquids A and B form an ideal solution in the entire composition range. At 350K, the vapor pressures of pure A and pure B are 7 × 10³ Pa and 12 × 10³ Pa, respectively. The composition of the vapor in equilibrium with a solution containing 40 mole percent of A at this temperature is:

For an ideal solution, Raoult's law is applicable:
PA = xA P°A
PB = xB P°B
where PA and PB are the partial pressures of A and B in the vapor phase, xA and xB are the mole fractions of A and B in the liquid phase, and P°A and P°B are the vapor pressures of pure A and pure B, respectively.
The total pressure is given by:
PTotal = PA + PB = xA P°A + xB P°B
The mole fraction of A in the vapor phase (yA) is given by:
yA = PA / PTotal = (xA P°A) / (xA P°A + xB P°B)
Given:
xA = 0.4 (40 mole percent of A)
xB = 1 - xA = 1 - 0.4 = 0.6
P°A = 7 × 10³ Pa
P°B = 12 × 10³ Pa
Substituting the values:
yA = (0.4 × 7 × 10³) / (0.4 × 7 × 10³ + 0.6 × 12 × 10³) = (2800) / (2800 + 7200) = 2800 / 10000 = 0.28
yB = 1 - yA = 1 - 0.28 = 0.72
Therefore, the composition of the vapor in equilibrium with a solution containing 40 mole percent of A at 350K is xA = 0.28 and xB = 0.72.