Enter the reactants information from the balanced equation to calculate excess reactant.

Excess Reactant Calculator

Find the limiting reagent, excess reagent remaining, and optional theoretical product yield.

Reaction-Based
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Use two reactants from a balanced equation. Enter masses in grams, molar masses in g/mol, and stoichiometric coefficients from the balanced reaction.
Reactant A
Reactant B
Optional product output

Excess Reagent Formula

The following relationship is used to calculate how much of a particular reagent remains in excess after a reaction, once you know how much of that same reagent was consumed. Variables:

ER = (TR - U)
  • ER is the excess amount remaining of the reagent you are tracking (mass or moles)
  • TR is the total (initial) amount of that reagent before reaction (same units as ER)
  • U is the amount of that same reagent that is used/consumed by the reaction (often found from stoichiometry after identifying the limiting reagent)

To calculate the excess remaining, subtract the amount used from the initial amount. In general chemistry problems, identifying which reagent is “in excess” and finding U requires the balanced chemical equation (stoichiometry).

What is an Excess Reagent?

An excess reagent, in chemistry, is a reactant present in more than the amount required by the balanced chemical equation. The reaction proceeds until the limiting reagent (the reactant that is completely consumed first) is used up; at that point, some of the excess reagent remains unreacted.

How to Calculate Excess Reagent?

The following steps outline how to calculate the excess remaining of a reagent using the relationship: ER = (TR - U).

  1. First, determine the initial amount of the reagent you are tracking (TR).
  2. Next, determine how much of that same reagent is consumed by the reaction (U). (This typically comes from stoichiometry after identifying the limiting reagent.)
  3. Subtract the amount used (U) from the initial amount (TR) using the formula ER = (TR - U).
  4. Finally, report the excess remaining (ER) in the same units as TR and U.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Initial amount (TR) = 50 g

Amount used (U) = 30 g

Excess remaining (ER) = 50 g − 30 g = 20 g