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 $M_B=0.3M_A$ where $M_B$ and $M_A$ are molecular weights of (B) and (A) respectively, the weight ratio of the solvent and solute are:

• A. $0.15$
• B. $5.7$
• C. $0.2$
• D. $4.0$

Answer 1

Answer: (B)

$5.7$

Using Raoult's law, relative lowering of vapour pressure is equal to the mole fraction of solute.

$X_A=5/100=0.05\Rightarrow X_B=0.95$

Now, ratio of weight of solvent to solute $=\frac{w_B}{w_A}=\frac{0.95\times M_B}{0.05\times M_A}=19\times 3/10=5.7$

Option B is correct.