Question

Nitrobenzene freezes at $278.98{}^{\circ }\text{C}$. If $0.25$ molal solution of a solute in nitrobenzene causes freezing point depression of $2{}^{\circ }\text{C}$, then $K_f$ (in ${\text{K kg mol}}^{-1}$) for nitrobenzene is:

• A. $2{\text{K kg mol}}^{-1}$
• B. $4{\text{K kg mol}}^{-1}$
• C. $8{\text{K kg mol}}^{-1}$
• D. $6{\text{K kg mol}}^{-1}$

Answer 1

The correct option is C $8{\text{K kg mol}}^{-1}$

Here depression in freezing point is $\Delta T_f=2\text{K}$.
$\therefore K_f=\frac{\Delta T_f}{m}=\frac{2}{0.25}=8{\text{K kg mol}}^{-1}$