Question

Equimolal solutions of $KCl$ and compound $X$ in water shows depressions in freezing point in the ratio of $4: 1$ . Assuming $KCl$ to be completely ionised, the compound $X$ in solution must

• A. dissociate to the extent of $50 \%$
• B. hydrolyse to the extent of $80 \%$
• C. dimerise to the extent of $50 \%$
• D. trimérise to the extent of $75 \%$

Answer 1

Answer: (D)

$\frac{\Delta T_{f}( KCl )}{\Delta T_{f}(X)}=\frac{4}{1}=\frac{i( KCl )}{i(X)}=\frac{2}{i(X)} $

$\Rightarrow i(X)=0.5$

For association of 3 molecules,

$i(X)=1-\left(1-\frac{1}{n}\right) \alpha=1-\left(1-\frac{1}{3}\right) \alpha=0.5 $

$\Rightarrow \alpha=0.75$