Problem : You have 4 moles of O2 and 2 moles of FeS2 and are given the following balanced equation:

4FeS2 +11O2→2Fe2O3 +8SO2    

Which reactant is the limiting reagent?

= 5.5 moles O2    

5.5 moles O2 are needed to completely react with 2 moles FeS2. Since we don't have this much O2, O2 is the limiting reagent.