In the formation of sulphur trioxide by the contact process, $2{\mathrm{SO}}_2+{\mathrm{O}}_2\rightleftharpoons 2{\mathrm{SO}}_3$, the rate of reaction was measured as $\frac{\mathrm{d}\left[{\mathrm{O}}_2\right]}{\mathrm{dt}}=3.0\times {10}^{-4} \mathrm{mol} {\mathrm{L}}^{-1} {\mathrm{s}}^{-1}$. 

The rate of reaction expressed in terms of ${\mathrm{SO}}_3$ will be