Question

0.5 molal aqueous solution of a weak acid $HX$ is 20% ionized. If $K_f$ for water is 1.86 $Kkgmo{l}^{-1}$, the lowering in freezing point of the solution is

• A. – 0.56 K
• B. – 1.12 K
• C. 0.56 K
• D. 1.12 K

Answer 1

The correct option is D

1.12 K

First up, let's calculate the van't Hoff factor $i$:

$i=(1-\alpha )+\alpha +\alpha =1+\alpha$

$⟹i=1+0.2=1.2$

We need to calculate the depression in freezing point. Pretty much everything needed is given and all we need to do is plug in the values into the following formula:

$\Delta T_f=i\times K_f\times m=1.2\times 1.86\times 0.5=1.12K$