If the pressure over the mixture at 300 K is reduced, at what pressure does the first vapour form? — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

The first vapour forms at the bubble point where the liquid composition is the initial total composition: $X_{\text{benzene}} = 0.5, X_{\text{toluene}} = 0.5$. The bubble point pressure is $P = P_{\text{benzene}}^\circ X_{\text{benzene}} + P_{\text{toluene}}^\circ X_{\text{toluene}} = 100(0.5) + 40(0.5) = 50 + 20 = 70 \text{ mm Hg}$. Therefore, correct answer is B.