Question

The relative lowering of vapour pressure of an aqueous solution containing a non volatile solute is $0.0125$. The molality of the solution is:

• A. $0.70$
• B. $0.30$
• C. $0.125$
• D. $0.07$

Answer 1

The correct option is A $0.70$

According to Raoult's law,

Relative lowering of vapour pressure $=$ mole fraction of solute.

$\left(X_B\right)=0.0125$

$\because X_B=\frac{m\cdot M_A}{1000+m\cdot M_A}$

Where m=molality,

$\therefore 0.0125=\frac{m\times 18}{1000+m\times 18}$

$12.5+0.225m=18$ m

$17.775m=12.5$

$m=\frac{12.5}{17.775}=0.70$.

Hence, option A is correct.