25.5 g of H₂O₂ solution on decomposition gave 1.68L of O₂ at STP. The percentage strength by weight of the solution is

Balanced equation for decomposition of H₂O₂

$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$

2mole H_2​O_2​ will produce 1 mol O₂​ 

At STP 1 mole O_2​ has volume = 22.4L
?????------------------------------------ At STP 1.68L of O_2​  

$\Rightarrow \frac{1.68\times 1}{22.4}=0.075 \mathrm{mole}$ of  O_2​ 

Moles of hydrogen peroxide is twice the moles of oxygen gas, therefore the mole of hydrogen peroxide is 0.15 moles (0.075 mol×2).

mass of H₂O₂ = 34 gms

Mass of H₂O₂ decomposed  

moles = $\frac{\mathrm{mass}}{\mathrm{molar} \mathrm{mass}}$

0.15 =$\frac{\mathrm{mass}}{34} \Rightarrow \mathrm{mass}=5.1\mathrm{gms}$

5.1 gms of H₂O₂ in 25.5 gms solution

100 gms of H₂O₂-------???    

=20%