A 20.0 mL solution containing 0.2 g impure $\mathrm{H}_2 \mathrm{O}_2$ reacts completely with 0.316 g of $\mathrm{KMnO}_4$ in acid solution. The purity of $\mathrm{H}_2 \mathrm{O}_2$ (in \%) is $\_\_\_\_$ (mol. wt. of $\mathrm{H}_2 \mathrm{O}_2=34$; mol. wt. of $\mathrm{KMnO}_4=158$ )
