The balanced chemical equation for the reaction between PCl5 and water is given below. If 3.45 moles of HCl are produced, how many moles of water reacted?
PCl5 + 4H2O -> H3PO4 + 5HCl

The coefficients of the substances give you the ratio of the number of moles.The 4 before the H2O and the 5 before the HCl tell you that for every 5 moles of HCl created, 4 moles of H2O had to react.Therefore:[tex](3.45mol HCl)(\frac{4 mol H2O}{5 mol HCl})[/tex]can be used to find the moles of H2O that react.[tex](3.45mol HCl)(\frac{4 mol H2O}{5 mol HCl}) = 2.76 mol H20[/tex]