I need some help with a stoichiometry problem from my basic Chem class. Here it is: For the reaction: 2S(s) + 3O2 -> 2SO2(g) How many moles of SO3 will be produced from 2.0 mol O2 and excess S? This seems like a very simple conversion from moles O2 to moles SO3 - to which I have an answer of 1.3 mol SO3, but I don't think it is correct (I have no answer key). I am getting confused with the "and excess S" part of the question and think that more needs to be done? Any help?