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.