0.5 molal aqueous solution of a weak acid $(\mathrm{HX})$ is $20 \%$ ionised. If $K_{f}$ for water i

Question

0.5 molal aqueous solution of a weak acid $(\mathrm{HX})$ is $20 \%$ ionised. If $K_{f}$ for water is $1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$, the lowering in freezing point of the solution is\

Professor PC Thomas Classes · Answer

$$\begin{array}{lcccc}
	ext{HX} & ightleftharpoons & 	ext{H}^+ & + & 	ext{X}^- \
1-\alpha & & \alpha & & \alpha \
\end{array} $$
Total $=1+\alpha$\ $	herefore \quad i=1+\alpha=1+0.2=1.2$\ $\Delta T_{f}=i 	imes K_{f} 	imes m=1.2 	imes 1.86 	imes 0.5=1.116 \mathrm{~K} \approx 1.12 \mathrm{~K}$\