Question

The vapour pressure of a solution of a non-volatile electrolyte (A) in a solvent (B) is $95 \%$ of the vapour pressure of the solvent at the same temperature. If $M_{B}=$ $0.3 M _{ A }$, 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. 4.0

Answer 1

Answer: (B)

$\frac{P^{o}-P_{s}}{P_{s}}=\frac{n}{N}=\frac{w \times M}{m \times W}$

or $\frac{P^{\circ}-\frac{95}{100} P^{\circ}}{\frac{95}{100} P^{\circ}}$

$=\frac{w \times 0.3 \,m}{W \times m}(M=0.3 \,m)$

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