A mixture contains 1 mole of volatile liquid A () and 3 moles of volatile liquid B (). If the soluti — Solutions and Colligative Properties Chemistry Question
Question
A mixture contains 1 mole of volatile liquid A ($P_A^\circ = 100 \text{ mm Hg}$) and 3 moles of volatile liquid B ($P_B^\circ = 80 \text{ mm Hg}$). If the solution behaves ideally, the total vapour pressure of the distillate is
Answer: B
💡 Solution & Explanation
Liquid phase: $X_A = 0.25, X_B = 0.75$. Total pressure $P = 100(0.25) + 80(0.75) = 25 + 60 = 85 \text{ mm Hg}$. Distillate composition (first vapour): $Y_A = 25/85 = 5/17$, $Y_B = 60/85 = 12/17$. Vapour pressure of this new liquid distillate $= P_A^\circ(5/17) + P_B^\circ(12/17) = 100(5/17) + 80(12/17) = (500 + 960)/17 = 1460/17 \approx 85.88 \text{ mm Hg}$. Therefore, correct answer is B.
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