解：設需要?$\mathrm {H}_{2}\mathrm {O}_{2}$?的質量為?$x。$??$ $?
?$2\mathrm {H}_2\mathrm {O}_2\xlongequal[ ]{\mathrm {MnO}_2}2\mathrm {H}_2\mathrm {O}+\mathrm {O}_2↑$?
68 32
?$x$? 3.2 g
?$\frac {68}{32}=\frac {x}{3.2 \mathrm {g}} ，$?解得?$x=6.8 \mathrm {g} $?
故雙氧水的質量?$=\frac {6.8 \mathrm {g}}{5\%}=136 \mathrm {g}$?
答：需要含過氧化氫?$5\%$?的雙氧水?$136 \mathrm {g}。$?
