What is the composition of last traces of vapours remained? — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

The last trace of vapour is in equilibrium with the bulk liquid at the bubble point ($X_A = 0.4$, $P = 0.52 \text{ bar}$). Using Raoult's law, $Y_A = \frac{P_A^\circ X_A}{P_{\text{total}}} = \frac{0.4 \times 0.4}{0.52} = \frac{0.16}{0.52} = \frac{16}{52} = \frac{4}{13}$. Therefore, correct answer is D.