Question

The vapour pressure of two pure liquids A and B which form an ideal solution are 1000 and 1600 torr respectively at 400 K. A liquid solution of A and B for which the mole fraction of A is 0.60 is contained in a cylinder by a piston on which the pressure can be varied. The solution use slowly vapourised at 400 K by decreasing the applied pressure. What is the composition of last droplet of liquid remaining in equilibrium with vapour?

• A. $X_A=0.30,X_B=0.70$
• B. $X_A=0.40,X_B=0.60$
• C. $X_A=0.70,X_B=0.30$
• D. $X_A=0.50,X_B=0.50$

Answer 1

The correct option is C $X_A=0.70,X_B=0.30$

$\begin{array}{c}\text{Here,}{Y}_A=\frac{P_A^0{X}_A}{P_A^0{X}_A+P_B^0(1-X_A)}⟹0.6=\frac{1000X_A}{1000X_A+1600(1-X_A)}\\ \therefore X_A=0.70X_B=1-X_A=0.30\end{array}$