That means we're going to produce two moles of carbon dioxide and four moles of oxygen. Now, it's exactly what I said here. Once our reaction runs to completion, two moles of methane and four moles of oxygen are used up to produce two moles of carbon dioxide and four moles of H 20. But notice we begin with six moles of oxygen. That means if only four moles of oxygen were used up, we have two moles of oxygen left over. And that means one of the reactants is completely used up.