If the pressure is reduced further, at what pressure does the last trace of liquid disappear? — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

The last trace of liquid disappears at the dew point, where the vapour composition is equal to the initial total composition: $Y_{\text{benzene}} = 0.5, Y_{\text{toluene}} = 0.5$. Dew point pressure $P = \frac{1}{(Y_{\text{benzene}} / P_{\text{benzene}}^\circ) + (Y_{\text{toluene}} / P_{\text{toluene}}^\circ)} = \frac{1}{(0.5 / 100) + (0.5 / 40)} = \frac{1}{0.005 + 0.0125} = \frac{1}{0.0175} = \frac{400}{7} \approx 57.14 \text{ mm Hg}$. Therefore, correct answer is A.