Zinc Sulfide reacts with oxygen according to the reaction:
2ZnS (s) + 3 O2(g) -> 2 ZnO (s) + 2 SO2 (g)

A reaction mixture contains 4.2 moles of zinc sulfide and 6.8 moles of oxygen. Once the reaction occurred as completely as possible, what amount in moles is left of the excess reactant?