10 g of bleaching powder on reaction with KI required 50 mL of 2N hypo solution. Thus, % of bleaching powder is

$$\begin{gathered} \underset{1\mathrm{mole}}{{\mathrm{CaOCl}}_2}+2\mathrm{KI}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{I}}_2+{\mathrm{CaCl}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{K}}_2{\mathrm{SO}}_4 \\ {\mathrm{I}}_2+2\underset{2\mathrm{moles}}{{\mathrm{S}}_2{\mathrm{O}}_3^{-2}}\to {\mathrm{S}}_4{\mathrm{O}}_6^{-2}+2{\mathrm{I}}^{-} \end{gathered}$$

No. of moles of hypo solution is 0.1 mole(50x0.1), so moles of bleaching powder in reaction is 0.05 mole =0.05×127=6.35gm

So % purity of solution is 63.5%.