The reactant that is supplied in an amount smaller than required by the stoichiometric relation for any of the other reactants to be consumed completely.

• A. Excess reactant
• B. Limiting reactant
• C. Catalyst
• D. Conjugate reactant

Answer: (B)

The situation describes the limiting reactant. In a reaction, reactants are consumed according to their stoichiometric ratios, and the one that is present in the smallest amount relative to what the equation requires will run out first. Once that reactant is exhausted, the reaction stops, even if others are still present in excess.

A quick way to spot it is to compare how many times the reaction can be "started" with each reactant given its available amount. For each reactant, divide the amount you have by its coefficient in the balanced equation; the smallest quotient points to the limiting reactant. For example, in a reaction where 1 mole of A reacts with 2 moles of B, having 3 moles of A and 7 moles of B means the reaction can proceed only 3 times (since A would be used up), so A is the limiting reactant and B would remain in excess.

That’s why the described reactant is the limiting one, not the excess reactant, a catalyst that isn’t consumed, or any nonstandard term like conjugate reactant.