Zinc reacts with excess hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂.

Rounds of the equation for a reactant $= \dfrac{\text{moles}}{\text{coefficient}}$. The smallest is the extent, and its reactant is limiting. Products: extent $\times$ coefficient. Excess left: moles $-$ extent $\times$ coefficient. Percent yield $= \dfrac{\text{actual}}{\text{theoretical}} \times 100$. At STP a mole of gas fills $22.7\ \text{L}$.