Question

The vapour pressure of a solvent decreases by $10mm$ of $Hg$ when a non-volatile solute was added to the solvent. The mole fraction of the solute in the solution is $0.2$ . What should be the mole fraction of the solvent if the decrease in vapour pressure is found to be $20mm$ of $Hg$ ?

• A. $0.8$
• B. $0.6$
• C. $0.4$
• D. $0.2$

Answer 1

Answer: (B)

Case $-1$
$P^0-P_s=10\Rightarrow X_B=0.2$
$RWP=X_B$
$\Rightarrow \frac{10}{P^{\circ }}=0.2\Rightarrow 50$
Case $-2$
$P^{\circ }-P_s^{{}^{\prime }}=20\Rightarrow X_A=?$
Now, $\frac{20}{P^{\circ }}=X_B$
$\Rightarrow \frac{20}{50}=X_B$
$\Rightarrow 0.4=X_B$
$\Rightarrow X_A=0.6$