Limiting Reagent / Limiting Reactant Calculator

How it works

For a balanced reaction aA + bB → cC, each reactant is first converted to moles. The calculator then compares the available reaction extent for each reactant using n / coefficient. The smallest extent identifies the limiting reagent because it would be used up first.

This mirrors the standard classroom workflow for limiting-reagent problems. You may start with grams, gas volumes, particle counts, or direct mole amounts, but the comparison only becomes meaningful after every reactant is expressed on the same mole basis. Once that is done, dividing by the stoichiometric coefficient tells you how far each reactant could push the balanced reaction. The reactant with the smallest possible extent sets the ceiling for the whole system, determines theoretical yield, and tells you how much of the other reactants remain in excess after the limiting amount is fully consumed.

• Limiting reagent: reactant with the smallest n / coefficient
• Theoretical yield: moles of product = coefficient × limiting extent
• Excess left over: initial moles − consumed moles

Assumptions and formulas

• Mass conversion: n = m / M where M is molar mass in g/mol
• Particles: n = particles / 6.02214076×10²³
• Gas volume: n = V / Vₘ using the selected molar volume reference
• Theoretical yield: n_product = coefficient_product × min(n_reactant / coefficient_reactant)

Formula-based molar masses use average atomic weights and simple parenthesis parsing. If you enter a label instead of a parseable formula, add a manual molar mass for any mass-based or mass-output conversion.

The calculator assumes the reaction is already balanced and that the stoichiometric coefficients are chemically correct. It does not account for incomplete conversion, parallel reactions, equilibrium limits, catalyst behavior, or non-ideal gas corrections. Those simplifications are normal for introductory stoichiometry and yield-estimation work, but they matter if you are comparing against real laboratory or industrial data.

Worked examples

Water formation: for 2H₂ + O₂ → 2H₂O, 4.00 g H₂ is about 1.984 mol and 32.0 g O₂ is exactly 1.000 mol. Comparing 1.984 / 2 and 1.000 / 1 shows both are nearly stoichiometric, so the reaction gives about 1.984 mol of water, or roughly 35.7 g.

Ammonia synthesis: for N₂ + 3H₂ → 2NH₃, 5.00 mol N₂ and 9.00 mol H₂ gives extents of 5.00 and 3.00, so H₂ is limiting and theoretical ammonia is 2 × 3.00 = 6.00 mol.

Propane combustion: for C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, 1.00 mol propane needs 5.00 mol oxygen. If only 160 g O₂ is available, that is exactly 5.00 mol, so the reactants are matched stoichiometrically.

Frequently asked questions

What is the difference between limiting reagent and limiting reactant?

They mean the same thing in standard stoichiometry problems. Different textbooks prefer one term or the other.

What if two reactants give the same limiting extent?

That means the reactants are present in stoichiometric proportion. In practice, the calculator marks each reactant within a small numerical tolerance of the minimum extent as limiting/fully used.

Can I calculate percent yield here?

This page is focused on limiting reagent, excess reactant, and theoretical yield. For percent yield and broader balanced-equation workflows, use the Stoichiometry Calculator.

Is this appropriate for equilibrium or side-reaction systems?

No. This is a stoichiometric completion calculator. It does not model equilibrium, kinetics, side products, or incomplete conversion.

In other words, the page tells you what the balanced equation predicts under ideal stoichiometric completion. That is exactly the right question for most homework and quick lab planning, but it should not be confused with a full reaction-engineering simulation.