The observed and normal molar masses of compound MX_2 are 65.6 and...

Normal molar mass of $\mathrm{MX}_2=164.0 \mathrm{~g} \mathrm{~mol}^{-1}$ Observed molar mass of $\mathrm{MX}_2=65.6 \mathrm{~g} \mathrm{~mol}^{-1}$$\begin{aligned} & \text{ Van't Hoff factor }(\mathrm{i})=\frac{\text{ Normal molar mass }}{\text{ Observed molar mass }} \\ & =\frac{164}{65.6}=2.5 \end{aligned}$If $\alpha$ is the degree of ionisation, then\begin{aligned} & \underset{1-\alpha}{\mathrm{MX}_2} \rightleftharpoons \mathrm{M}_\alpha^{2+}+\underset{2 \alpha}{2 \mathrm{X}^{-}} \\ & \mathrm{i}=1-\alpha+\alpha+2 \alpha=1+2 \alpha \\ & 1+2 \alpha=2.5 \\ & \alpha=0.75 \end{aligned}$$\therefore$ Percent degree of ionisation $=75 \%$