Question

A and B form an ideal solution at 298 K, with x_A^(l)=0.55, P*_A = 0.1296 atm and P*_B = 0.0758 atm.

a. Calculate the partial pressures of A and B in the gas phase.

b. A portion of the gas phase is removed and condensed in a separate container. Calculate the partial pressures of A and B in equilibrium with this liquid sample at 298 K.

Answer 1

Roult's law for ideal solution is,

P_A = x_A (l) P_A^* = 0.55 x 0.1269 = 0.07 atm.

P_B = x_B (l) P_B^* = (1 - x_A (l)) P_B^* = (1-0.55) x 0.0758 = 0.0341 atm.

a. The partial pressures of A and B in the gas phase is 0.07 atm. and 0.0341 atm.

b. When a portion of the gas phase is removed the partial vapor pressure decreases above the liquid but it is not in equilibrium now. But when it comes to equilibrium it reaches the same partial pressure as before that is the partial pressures of A and B in equilibrium with this liquid sample is 0.07 atm. and 0.0341 atm.