In order to use all of the starting magnesium and produce the maximum amount of hydrogen gas possible, a 1:2 ratio of magnesium to hydrochloric acid is needed. This is represented in the chemical equation below by adding a coefficient in front of the HCl.

Coefficients tell you how many moles of each chemical are needed for the reaction to take place most efficiently. If there is no coefficient present, then it is assumed to be 1. For the reaction above, you will need 1 mole of Mg with 2 moles of HCl to yield 1 mole of MgCl₂ and 1 mole of H₂.