The gas phase decomposition of hydrogen peroxide at 400 °C

H2O2(g)H2O(g) + ½ O2(g)

is second order in H2O2 with a rate constant of 0.650 M-1 s-1.

If the initial concentration of H2O2 is 8.00×10-2 M, the concentration of H2O2 will be 1.70×10-2 M after
seconds have passed.