$H_2O_2$ on reaction with $PbS$ gives

There is a sample of $10$ volume of hydrogen peroxide solution. Calculate its strength in percentage (%).

Assertion $:$ Amount of $H_2O_2$ in a sample can be determined by treating $H_2O_2$ solution with $KI$ and followed by titration of liberated $I_2$ with sodium thiosulphate.
Reason $:$ In this titration,an intense blue colour appears which disappears at the end point.

Hydrolysis of one mole of peroxodi-sulphuric acid produces

The concentration of commercial $H_2O_2$ is $3.125 \ M$,then its grade is $..........$ volume.

The decomposition of $H_2O_2$ can be prevented by adding which of the following?