Question

Vapour pressure of pure benzene is 119 torr and 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. $0.205$
• C. $0.237$
• D. $0.435$

Answer 1

Answer: (C)

$0.237$

Partial pressure= Mole fraction $\times$ Total pressure

$\Rightarrow {X_A{P}_A}^o=Y_A{P}_T$ $Y_A$= Mole fraction in vapour phase

$\Rightarrow P_T={X_A{P}_A}^o+X_B$

$=(0.5\times 37)+(0.5\times 119)$

$P_T=78$ torr

Now, $Y_A=\frac{{X_A{P}_A}^o}{P_T}=\frac{0.5\times 37}{78}=0.237$

Hence, the correct option is $C$