Chemical Kinetics

For the reaction, $2 \mathrm{SO}_2+\mathrm{O}_2 \rightleftharpoons 2 \mathrm{SO}_3$, the rate of disappearance of $\mathrm{O}_2$ is $2 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1}$. The rate of appearance of $\mathrm{SO}_3$ is