Question

The vapour pressure of a solution of a non-volatile electrolyte B in a solvent A is 95% of the vapour pressure of the solvent at the same temperature. If the molecular weight of the solvent is 0.3 times the molecular weight of solute, the weight ratio of the solvent and solute are:

• A. $0.15$
• B. $6.0$
• C. $0.2$
• D. $4.0$

Answer 1

The correct option is B $6.0$

The exact mathematical expression of Raoult's law is $\frac{P^0-P_s}{P^0}=\frac{wM}{Wm}$

Here, $P_s$ and $P^0$ are the vapour pressures of the solution and pure solvent respectively.

m and M are the molecular weights of solute and solvent respectively.
w and W represent the mass of solute and solvent respectively.

$P_s=0.95P^0$

$M=0.3m$

Substitute values in the expression of Raoult's law.

$\frac{P^0-0.95P^0}{P^0}=\frac{w\times 0.3m}{Wm}$

$\frac{W}{w}=\frac{0.3}{0.05}=6$

Hence, the correct option is $\text{B}$.