How many moles of water would it take to produce 27 moles of oxygen gas?

The reactants have four hydrogen atoms and four oxygen atoms. On the other hand, the products have just two hydrogen atoms and three oxygen atoms. So, to balance the chemical equation, there should be two more hydrogen atoms and one more oxygen atom. This imbalance can be corrected by placing the number 2 in front of H2O on the reactants' side.