Question

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

Answer 1

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:
$(3.45mol HCl)(\frac{4 mol H2O}{5 mol HCl})$
can be used to find the moles of H2O that react.

$(3.45mol HCl)(\frac{4 mol H2O}{5 mol HCl}) = 2.76 mol H20$