when copper combines with oxygen, copper (II) oxide is formed. if 7.0 mol of copper reacts with 4.0 mil of oxygen, what amount of the excess reactant remains?

Answer: I believe that there are. 5 moles of O₂ left over.

Explanation: O₂ + 2 Cu = 2 CO₂ + 2 H₂O. With 7 moles of Cu you can get 7 moles of CuO. But, with 4 moles of O₂ you can get 8 moles of CuO, so O₂ is the limiting reactant. Thus, you would have to use 7 moles of Cu in order to determine the moles of O₂ and you get 3.5 moles of O₂. 4 moles - 3.5 moles =.5 moles