The volume of 40 vol. H₂O₂ solution required to prepare 2L. of O₂ at STP is

40 volume strength of H₂O₂ solution means,
1ml of H₂O₂ solution   $\stackrel{gives}{\to }$  40 ml of "O₂"
??     of H₂O₂ solution $\stackrel{gives}{\to }$2L=2000ml 0f ${\mathrm{O}}_2$

$\frac{2000\mathrm{ml}\times 1\mathrm{ml}}{40\mathrm{ml}}$= 50ml of H₂O₂ solution
50ml of 40 vol strength of H₂O₂ solution at STP produces 2L (or) 2000ml of O₂ .