Suppose you initially have equal amounts of the reactants: 1 g of zinc (65.38 g/mol) and 1 g of HCl (36.46 g/mol). The reaction would be:
Zn + 2 HCl → ZnCl₂ + H₂
Moles Zn: 1/65.38 = 0.0153 mol
Moles HCl: 1/36.46 = 0.0274 mol
Moles of HCl needed:
0.0153 mol Zn (2 mol HCl/1 mol Zn) = 0.0306 moles HCl needed
Since the available moles of HCl is lesser than the theoretical amount of HCl needed, then the limiting reactant is HCl in this reaction.
