Limiting reactant is defined as

• A. The reactant that is completely consumed and limits product amount
• B. The reactant that is in excess after reaction completes
• C. The reactant not present in the reaction
• D. The reactant with the higher price

Answer: (A)

The limiting reactant is the substance that runs out first during a reaction and thus sets the maximum amount of product that can be formed. In a reaction, reactants are used in fixed stoichiometric ratios; when the limiting reactant is consumed, the reaction stops even if other reactants are still present in excess. The amount of product produced is determined by how much of this limiting reagent you started with, because you can’t make more product than the stoichiometry allows based on that reagent.

For example, in A + 2B → AB2, if you begin with 3 moles of A and 4 moles of B, you would need 6 moles of B to consume all 3 moles of A, so B runs out first. This makes B the limiting reactant, and you can form only 2 moles of AB2 (using 4 B and 2 A), leaving 1 A unreacted. The remaining A is in excess, not limiting.

The other statements describe either the leftover reactant that’s in excess, a reactant that isn’t present, or an irrelevant factor like price, none of which define the concept of a limiting reactant.