Question

The vapour pressure of a solution of a non-volatile electrolyste (A) in a solvent (B) is 95% of the vapoure 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 solute are :

• A. $0.15$
• B. $5.7$
• C. $0.2$
• D. $4.1$

Answer 1

Answer: (B)

$5.7$

The mass ratio of the solvent and solute is 5.7
Vapor pressure is related to mass of solute and solvent by following formula :

$\frac{P^0-P_S}{P_S}=\frac{n}{N}=\frac{w\times M}{m\times W}$

or $\frac{P^0-\frac{95}{100}{P}^0}{\frac{95}{100}{P}^0}=\frac{w\times 0.3m}{W\times m}(M=0.3m)$

or $\frac{W}{w}=5.7$