The strength of 11.2 volume solution of ${\mathrm{H}}_2{\mathrm{O}}_2$ is : [Given that molar mass of H=1 g ${\mathrm{mol}}^{-1}\mathrm{and}\mathrm{O}=16\mathrm{g}{\mathrm{mol}}^{-1}$]

$\begin{array}{rc}2{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)\to 2{\mathrm{H}}_2\mathrm{O}+& {\mathrm{O}}_2\left(\mathrm{g}\right)\\ & \text{ 1mole or }\\ & 22.4\text{ Lat STP }\end{array}$

By definition of 11.2 volume solution of ${\mathrm{H}}_2{\mathrm{O}}_2$,
Moles of   gas formed=11.2 L or $\frac{1}{2}$ mole 
By stoichiometry, moles of ${\mathrm{H}}_2{\mathrm{O}}_2$ present in 1 L solution=1 mole
$\therefore$ Mass of ${\mathrm{H}}_2{\mathrm{O}}_2$ in 1L solution=1x Molar mass of ${\mathrm{H}}_2{\mathrm{O}}_2$
                                                                        =1 x 34 = 34 gm
$\mathrm{\%}\frac{\mathrm{w}}{\mathrm{v}}\text{ strengh }=\frac{34}{1000}\times 100=3.4\mathrm{\%}$

Alternatively

volume strength = % strength x $\frac{10}{3}$