Which term refers to the reactant that is completely consumed first in a reaction?

• A. Stoichiometry
• B. Limiting reagent
• C. Percent yield
• D. Boyle's Law

Answer: (B)

Limiting reagent is the reactant that is completely consumed first and thus determines the maximum amount of product that can form. In a reaction, reactants react in fixed amounts given by the balanced equation, so you can have some leftover of one reactant while the other is used up. The one that runs out first sets the limit on how far the reaction can go, fixing the theoretical yield of product. The other terms don’t describe which reactant runs out first: stoichiometry deals with the quantitative relationships between reactants and products, percent yield compares the actual product obtained to that theoretical maximum, and Boyle's Law relates pressure and volume of gases at constant temperature.