How does a limiting reactant determine how much product can form?

In a chemical reaction, the limiting reactant (or limiting reagent) is the substance that is completely consumed first. Because matter is conserved and reactions follow specific stoichiometry—the fixed ratio of reactants and products—the amount of product that can be formed is strictly dictated by the quantity of this reactant available.1

Once the limiting reactant is exhausted, the reaction must stop, even if other reactants remain in excess. For example, if a reaction requires a specific ratio of two elements, such as iron and sulfur, and one is provided in a smaller relative amount than the other, the smaller amount determines the maximum yield of the product; any excess of the other reactant will simply remain unreacted. Understanding this relationship is essential for calculating mass relationships in chemical equations using the concept of the mole.1