# Stoichiometry problem

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?

CAF1119 said:
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?
I'll asume that was just a typo and should be SO3 as the product instead of SO2.

CAF1119 said:
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?

Your answer is correct. The fact that S is in excess just means that all 2.0 moles of O2 will react with the S producing SO3. You'll end up with an unknown amount of excess S after the reaction has taken place, but you know that all 2.0 moles of O2 have reacted.

Say the problem read like this: How many moles of SO3 will be produced from 2.0 mol O2 and 1.0 mol S?

In this case, you must find which reactant is the limiting reactant and which reactant is in excess. Using the chemical equation (2S(s) + 3O2 -> 2SO3(g)), you can see that for every 2 moles of S, 3 moles of O2 react. Since you have only 1.0 mol S, you know that only 1.5 moles O2 will react. Therefore you will have 0.5 mole O2 left over (2.0-1.5=0.5). O2 is in excess and S is the limiting reactant. Since S is the limiting reactant, all 1.0 mol S reacts to form 1.0 mol SO3.