A cylinder fitted with a movable piston initially contains $2.00 \mathrm{~mol}$ $\mathrm{O}_{2}(g)$ and an unknown amount of $\mathrm{SO}_{2}(g)$. The oxygen is known to be in excess. The density of the mixture is $0.8000 \mathrm{~g} / \mathrm{L}$ at some $T$ and $P$. After the reaction has gone to completion, forming $\mathrm{SO}_{3}(g)$, the density of the resulting gaseous mixture is $0.8471 \mathrm{~g} / \mathrm{L}$ at the same $T$ and $P$. Calculate the mass of $\mathrm{SO}_{3}$ formed in the reaction.