A $0.500\mathrm{g}$ sample of bleaching powder is treated with excess dilute acid and excess KI. The iodine liberated needs $49.3\mathrm{mL}$ of $0.100\mathrm{M}$ $\mathrm{Na}{}_2\mathrm{S}{}_2\mathrm{O}{}_3$ ($\mathrm{I}{}_2+2\mathrm{S}{}_2\mathrm{O}{}_3{}^{2-}\to 2\mathrm{I}{}^{-}+\mathrm{S}{}_4\mathrm{O}{}_6{}^{2-}$). Taking $\mathrm{Cl}=35.5$, calculate the percentage of available chlorine by mass (as $\mathrm{Cl}{}_2$) in the sample. Give the answer to the nearest integer.

Numerical answer type, answer in %, tolerance 0.5.