In many situations one of the reactants is present in excess therefore some of this reactant is left over on completion of the reaction.
For Example, consider the combustion of hydrogen.
2H2(g) + O2(g) → 2H2O(g)
Suppose that 2 moles of H2 and 2 moles of O2 are available for reaction. It follows from the equation that only 1 mole of O2 is required for complete combustion of 2 moles of H2 ; 1 mole of O2 will, therefore, be left over on completion of the reaction. The amount of the product obtained is determined by the amount of the reactant that is completely consumed in the reaction. This reactant is called the limiting reagent.
