A gaseous mixture of two substances A and B, under a total pressure of 0.8 atm is in equilibrium with an ideal liquid solution. The mole fraction of substance A is 0.5 in the vapour phase and 0.2 in the liquid phase. The vapour pressure of pure liquid A is _____ atm. (Nearest integer)

We have a gaseous mixture of A and B in equilibrium with an ideal liquid solution at a total pressure of $$P = 0.8$$ atm. The mole fraction of A in the vapour phase is $$y_A = 0.5$$ and in the liquid phase is $$x_A = 0.2$$.

By Raoult's law for an ideal solution, the partial pressure of A in the vapour phase equals:

where $$P_A^0$$ is the vapour pressure of pure liquid A.

Also, from Dalton's law of partial pressures, the partial pressure of A in the vapour is:

Now using Raoult's law:

Hence, the correct answer is 2.