Propose a reaction mechanism that explains the rate law for the formation of dinitrogen pentoxide
First step - the rate determining step or the slow step (activation energy is high).
$$\ce{NO(g) + NO2(g) -&gt; N2O3(g)}$$

2nd step - the fast step.
$$\ce{N2O3(g) + O2(g) -&gt; N2O5(g)}$$

Since the rate determining step does not include $\ce{O2(g)}$, the rate is independent of the initial $\ce{O2(g)}$ concentration.