A cylinder fitted with a movable piston initially contains 2.00 moles of O₂(g) and an unknown amount of SO₂(g). The oxygen is known to be in excess. The density of the mixture is 0.8000 g/L at some T and P. After the reaction has gone to completion, forming SO₃(g), the density of the resulting gaseous mixture is 0.8471 g/L at the same T and P. Calculate the mass of SO₃ formed in the reaction.