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 appearance of $ \mathrm{SO}_{3} $ is