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? Assume that the temperature is constant ${25}^{\circ }C$. (Given Vapour Pressure Data at ${25}^{\circ }C$, benzene $=12.8kPa,$ toluene $=3.85kPa$)

• A. The vapour will contain a higher percentage of benzene
• B. The vapour will contain a equal amount of benzene and toulene
• C. The vapour will contain a higher percentage of toulene
• D. Not enough information is given to make a prediction

Answer 1

Answer: (A)

$A\to benzene,B\to toluene$

In $1:1$ molar mixture of $A$ and $B$

$\therefore X_A=\frac{1}{2},X_B=\frac{1}{2}$

$P=P_A^o{X}_A+P_B^o{X}_B$

$P=12.8\times \frac{1}{2}+3.85\times \frac{1}{2}=8.325kPa$

$Y_A=\frac{P_A^o{X}_A}{P}=\frac{12.8\times \frac{1}{2}}{8.325}=0.768$

$\Rightarrow Y_B=1-Y_A=\mathrm{1.0.768}=0.232$

So the vapour will contain higher percentage of benzene.

Option A is correct.