The gas phase decomposition of

hydrogen peroxide at $400C$

$H_2{O}_2(g)H_{2}O(g)+{?}^{\frac{1}{2}}{O}_2(g)$

is second order in $H_2{O}_2$ with a rate

constant of $0\mathrm{.}650\frac{1}{M*s}.$

If the initial concentration of $H_2{O}_2$ is

$8\mathrm{.}90{10}^{-2}M,$ the concentration of

$H_2{O}_2$ will be $1\mathrm{.}87{10}^{-2}M$ after

seconds have passed.