Question

What is the mole fraction of toluene in the vapour phase at $30^{0}C$ above a benzene-toluene solution. The mole fraction of benzene in the solution phase is 0.400?
$\left(P_B^0=119\text{torr}and{P}_T^0=37.0\text{ torr}\right)$ , B stands for benzene and T stands for toluene

• A. 0.237
• B. 0.367
• C. 0.428
• D. 0.318

Answer 1

Answer: (D)

$P=X_A{P}_A^0+X_B{P}_B^0$
Total pressure of the sol. is given by
$P_{Total}=X_B{P}_B^0+X_T{P}_T^0$
$=0.4\times 119+0.6\times 37$
$=47.6+22.2=69.8torr$
Applying Dalton's law for mole fraction in vapour phase
$Y_B=\frac{P_B}{P_{Total}}=\frac{47.6}{69.8}=0.68194$
$Y_T=1-0.6819=0.318$