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The concept of a limiting reactant is crucial in stoichiometry, as it determines the maximum amount of product that can be formed in a chemical reaction. The limiting reactant is defined as the reactant that is completely consumed first during the reaction. Once this reactant is used up, the reaction cannot proceed any further, which directly limits the formation of products.

In a balanced chemical equation, the stoichiometric coefficients indicate the proportions in which reactants combine to produce products. If one reactant is present in a smaller amount than what is required based on these proportions, it will be the reactant that is exhausted first and, thus, limits how much product is produced. For example, if you are mixing reactants A and B in a reaction where the stoichiometry is 1:2, having only one mole of A and two moles of B means that A will run out first, making it the limiting reactant.

In contrast, the other options focus on different aspects of a chemical reaction that do not define a limiting reactant. The excess reactant, while it plays a role in determining the reaction rate, does not define the limiting reactant. A catalyst, while it does increase the speed of a reaction,