Molarity $=\frac{\mathrm{Volume} \mathrm{strength} \mathrm{of} {\mathrm{H}}_2{\mathrm{O}}_2}{\mathrm{Molar} \mathrm{volume} \mathrm{of} \mathrm{oxygen} \mathrm{gas} \mathrm{at} \mathrm{STP}}\times 2$

Molarity $=\frac{50}{11.35}$

The strength of the solution$=\mathrm{Molarity}\times \mathrm{Molecular} \mathrm{weight} \mathrm{of} {\mathrm{H}}_2{\mathrm{O}}_2$

$\therefore$ Strength in $\mathrm{gm}/\mathrm{L}=\frac{50}{11.35}\times 34$.