The reactant that gives the lowest yield by limiting the amount of product is the limiting reactant. The limiting reactant is the reactant that is entirely consumed in a chemical reaction, thereby limiting the amount of product that can be formed. In a reaction, if one reactant is present in a lesser mole ratio compared to the other reactants, it will be completely consumed, and the reaction will stop when this reactant is used up. The other reactants will be left in excess. The limiting reactant determines the maximum amount of product that can be obtained from the reaction, and therefore, it gives the lowest yield.