Question

Vapour pressure of pure benzene is $119$ torr and that of toluene is $37.0$ torr at the same temperature. Mole fraction of toluene in vapour phase which is in equilibrium with a solution of benzene and toluene having a mole fraction of toluene $0.50$, will be :

• A. $0.137$
• B. $0.237$
• C. $0.435$
• D. $0.205$

Answer 1

Answer: (B)

$P_A=P_A^0\times x_A=$ total pressure $\times y_A$
$P_B=P_B^0\times x_B=$ total pressure $\times y_B$
where $x$ and $y$ represents mole fraction in liquid and vapour phase respectively.
$\frac{P_B^0{x}_B^0}{P_A^0{x}_A}=\frac{y_B}{y_A};\frac{P_B^0\left(1-x_A\right)}{P_A^0{x}_A}=\frac{1-y_A}{y_A}$
on putting values $\frac{119\left(1-0.50\right)}{37\times 0.50}=\frac{1-y_A}{y_A}$
on solving $y_A=0.237$