In other words, 1 mol of this hydrocarbon reacts with seven over two moles of diatomic oxygen to produce two moles of carbon dioxide and three moles of water. We can never say 1 gram of this car react with seven over 2 grams. That makes no sense. These coefficients represent moles, molecules or atom, never any mass. So whenever a reaction is set to run to completion, what that basically means is that one of the reactants is completely used up. It's completely depleted.