In chemical quantities, the term excess refers to

• A. More than what you need
• B. Exactly what you need
• C. Less than you need
• D. No amount

Answer: (A)

Excess means you have more of a reactant than is needed to react completely with the other reactant(s). In a reaction, one reactant is used up first—the limiting reagent—while any leftover amount of the other reactant is the excess. For example, if a reaction needs 2 moles of hydrogen to react with 1 mole of oxygen, but you start with 3 moles of hydrogen and 1 mole of oxygen, all the oxygen is consumed and 1 mole of hydrogen remains. That leftover hydrogen is the excess. If you had exactly the right amounts (no leftover), there would be no excess; having less would prevent the reaction from using all the available reactant.