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(a) The limiting reactant is the reactant that runs out first in a chemical reaction, causing the reaction to stop and not produce more products. The excess reactant is the reactant present in a greater quantity than necessary for the stoichiometric balance and remains partially unused when the reaction is completed. (b) The limiting reactant determines the amount of product formed in a reaction because once it is consumed, the reaction stops, and no more product can be formed. The reaction requires the presence of all reactants in their stoichiometric quantities for it to occur. (c) The choice of the limiting reactant should be based on the number of initial moles because the stoichiometry of a reaction depends on the mole ratios of the reactants, not on their masses. Using initial moles enables calculations of mole ratios directly from the balanced chemical equation and helps determine which reactant will run out first, leading to the correct identification of the limiting reactant. The initial mass in grams may not provide accurate information about the reactant's stoichiometry within the reaction, while the number of initial moles does.