Question

A solvent X freezes at ${240}^{\circ }\text{C}$. If $0.25$ molal solution of a non volatile solute in pure X causes freezing point depression of $3^{\circ }\text{C}$, then $K_f$ (in ${\text{K kg mol}}^{-1}$) for solvent X is:

Assume complete association/dissociation of solute.

• A. $120{\text{K kg mol}}^{-1}$
• B. $20{\text{K kg mol}}^{-1}$
• C. $12{\text{K kg mol}}^{-1}$
• D. $16{\text{K kg mol}}^{-1}$

Answer 1

The correct option is C $12{\text{K kg mol}}^{-1}$

Here depression in freezing point is $\Delta T_f=3^{\circ }C=3\text{K}$.
$\therefore K_f=\frac{\Delta T_f}{m}=\frac{3}{0.25}=12{\text{K kg mol}}^{-1}$