Question

The vapour pressure of a solution of a non-volatile electrolyte $\text{a}$. in a solvent $\text{b}$. is 95% of the vapour pressure of the solvent at the same temperature. If $M_B=0.3M_A$, where $M_B$ and $M_A$ are molar masses of $B$ and $A$ respectively, the mass ratio of the solvent and the solute are:

• A. 0.015
• B. 5.7
• C. 0.2
• D. 4

Answer 1

The correct option is B 5.7

Relative lowering of a vapour pressure is a colligative property given by
$\frac{P_A^0-P_A}{P_A^0}=X_B$
where $P_A^0$ is original vapour pressure of solvent A
$P_A$ is observed vapour pressure of a of solution and $X_B$ is mole fraction of B
$X_B=\frac{P_A^0(1-0.95)}{P_A^0}=0.05$
for mass ratio
$\frac{0.95M_B}{0.05M_A}=massratio$
$=\frac{0.95\times 0.3M_A}{0.05M_A}$
$=5.7$