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# What is the composition of the vapour which is in equilibrium at $30^{\large\circ}C$ with a benzene-toluene solution with a mole fraction of benzene of 0.400? ($P_B^0$ = 119 torr and $P_T^0$ = 37.0 torr)

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## 1 Answer

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Total pressure of the solution is given by
$P_T = X_BP_B^0 + X_T P_T^0$
$\Rightarrow 0.4 \times 119 + 0.6 \times 37$
$\Rightarrow 47.6 + 22.2$
$\Rightarrow 69.8 torr$
Applying Dalton’s law for mole fraction in vapour phase, $Y_B=\large\frac{P_B}{P_T}=\frac{47.6}{62.4}$$=0.763$
Hence (A) is the correct answer.
answered Jun 9, 2014