Limiting Reagent Calculator
Identify the limiting reagent and excess reagent for a two-reactant reaction.
How It Works
Enter the moles of two reactants and their stoichiometric coefficients from the balanced equation. The calculator divides each reactant's moles by its coefficient to find the mole ratio. The reactant with the smaller ratio is the limiting reagent — it runs out first. The calculator also shows how many moles of the excess reactant remain unreacted.
Formula
Worked Example: Iron and Oxygen
Reaction: 4 Fe + 3 O₂ → 2 Fe₂O₃
- You have 6 mol Fe and 3 mol O₂
- Ratio for Fe: 6 / 4 = 1.5 | Ratio for O₂: 3 / 3 = 1.0
- O₂ has the smaller ratio → O₂ is the limiting reagent
- Fe consumed = 3 mol O₂ × (4/3) = 4 mol; Fe remaining = 6 − 4 = 2 mol excess
- Fe₂O₃ produced = 3 mol O₂ × (2/3) = 2 mol
Why Identifying the Limiting Reagent Matters
In a laboratory or industrial synthesis, you almost never have reactants in perfect stoichiometric ratios. The limiting reagent sets the ceiling on how much product you can obtain, no matter how much excess of the other reagent you add. Once you identify it, you can calculate theoretical yield and percent yield with confidence. Deliberately using an excess of one reagent (often the cheaper or safer one) is a common strategy to drive a reaction toward completion.
Connecting to Theoretical Yield
After finding the limiting reagent and the moles of product it produces, multiply by the product's molar mass to get the theoretical yield in grams. Use the Theoretical Yield Calculator to perform that final step, then compare to your actual measured mass with the Percent Yield Calculator.
Frequently Asked Questions
What is the limiting reagent?
The limiting reagent is the reactant that is completely consumed in a chemical reaction, setting the maximum amount of product that can form. All other reactants are present in excess relative to it.
How do I identify the limiting reagent?
Divide the moles of each reactant by its stoichiometric coefficient from the balanced equation. The reactant with the smallest result is the limiting reagent — it runs out first.
What is the excess reagent?
The excess reagent is the reactant that remains after the limiting reagent is fully consumed. Its leftover amount (in moles) = initial moles − moles consumed based on the limiting reagent.
Can a reaction have more than two reactants?
Yes. For three or more reactants, apply the same ratio method to each: divide moles by coefficient, then identify the smallest value. This calculator handles two reactants; for more complex reactions, repeat the comparison pairwise.
Does the limiting reagent change if I change the amounts?
Yes. The limiting reagent depends on the ratio of moles you actually use, not the compounds themselves. Adding more of the current limiting reagent can change which reactant becomes limiting.