Question 56
The vapour pressure of pure benzene and methyl benzene at $$27°C$$ is given as 80 Torr and 24 Torr, respectively. The mole fraction of methyl benzene in vapour phase, in equilibrium with an equimolar mixture of those two liquids (ideal solution) at the same temperature is ______ $$\times 10^{-2}$$ (nearest integer)
Correct Answer: 23
Solution
Equimolar mixture: $$x_1 = x_2 = 0.5$$.
By Raoult's law: $$P_{benzene} = 0.5 \times 80 = 40$$ Torr. $$P_{toluene} = 0.5 \times 24 = 12$$ Torr.
Total pressure = 52 Torr.
Mole fraction of toluene in vapour = $$12/52 = 3/13 = 0.2307... \approx 23 \times 10^{-2}$$.
The answer is $$\boxed{23}$$.
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