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Question:

Which of the following statements about the composition of the vapour over an ideal 1:1 molar mixture of benzene and toluene is correct?

The vapour will contain an equal amount of benzene and toluene

The vapour will contain a higher percentage of benzene

The vapour will contain a higher percentage of toluene

Not enough information is given to make a prediction

Solution:

A → benzene, B → toluene
In 1:1 molar mixture of A and B ∴ XA = 1/2, XB = 1/2
P = PoAXA + PoBXB
P = (12.8 × 1/2) + (3.85 × 1/2) = 8.325 kPa
YA = PoAXA / P = (12.8 × 1/2) / 8.325 = 0.768 → YB = 1 - YA = 1 - 0.768 = 0.232
So the vapour will contain a higher percentage of benzene.
Option B is correct.