1. The problem statement, all variables and given/known data Sulfur dioxide reacts with the oxygen in humid air to produce sulfur trioxide. After the reaction, how many moles are left of: SO2, O2 and how many moles of SO3 were created? 2. Relevant equations 2 SO2 + O2 --> 2 SO3 3. The attempt at a solution The limiting reactant is SO2, so we know that it will fully react and we'll end up with 0 moles of SO2. From previous steps, I know that I have 0.1021 moles of SO2, and 0.0857 moles of O2. I also know that after the reaction, I will have 0.03474 moles of O2 left. I can't, for the life of me, figure out how many moles of SO3 I'll have at the end. Shouldn't there be some conservation of mass? I keep finding that I'll have 0.1021 moles of SO3 at the end, which is the same amount as the SO2. Is this correct? I seem to think that the reaction with O2 will have changed the number of moles.