Question

Liquids A and B form an ideal solution and the former has stronger intermolecular forces. If $x_A$ and $x′_A$ are the mole fractions of A in the solution and vapour in equilibrium, then

• A. $ \frac{x'_{A}}{x_{A}} = 1 $
• B. $ \frac{x'_{A}}{x_{A}} > 1 $
• C. $ \frac{x'_{A}}{x_{A}} < 1 $
• D. $ x'_{A} + x_{A} = 1 $

Answer 1

Answer: (C)

$x'_{A} = \frac{p_{A}}{p} = \frac{p^{\circ}_{A}x_{A}}{p} \Rightarrow \frac{x'}{x_{A}} = \frac{p^{\circ}_{A}}{p}^{\circ} < 1$
(since liquid A is less volatile, $p^{\circ}_{A} < p$)