Limiting Reagent Calculator
Determine the limiting reagent, excess reagent, and theoretical yield given moles (or mass + molar mass) of two reactants and their stoichiometric coefficients.
Reactant A
Reactant B
Product
Formulas Used
Moles from mass:
n = m / M (mol = g ÷ g·mol⁻¹)
Identifying the limiting reagent:
Compare the mole-to-coefficient ratios for each reactant:
ratioA = nA / a | ratioB = nB / b
The reactant with the smaller ratio is the limiting reagent.
Moles of excess reagent required:
nB,required = nA × (b / a) (and vice-versa)
Moles of excess reagent remaining:
nexcess,remaining = nexcess,available − nexcess,required
Percent excess:
% excess = [(navailable − nrequired) / nrequired] × 100
Theoretical yield (moles of product):
nP = (nlimiting / coefflimiting) × coeffP
Theoretical yield (mass of product):
mP = nP × MP
where: a, b = stoichiometric coefficients of A and B; coeffP = stoichiometric coefficient of product; M = molar mass.
Assumptions & References
- The reaction goes to completion (100 % conversion of the limiting reagent).
- Only two reactants and one product are considered; extend the ratio method for more complex reactions.
- Masses and molar masses must be positive; a mass of 0 means that reactant is entirely absent and is automatically the limiting reagent.
- Stoichiometric coefficients must be positive real numbers (fractional coefficients are allowed).
- Theoretical yield assumes no side reactions and perfect stoichiometry.
- Method follows standard general-chemistry stoichiometry: Zumdahl & Zumdahl, Chemistry, 10th ed., Cengage, 2018, Ch. 9.
- Molar masses can be obtained from the IUPAC periodic table: iupac.org.