Chemistry · Stoichiometry
Limiting reagent calculator.
Enter the balanced-equation coefficients, molar masses and grams. Get the limiting reactant, theoretical yield, leftovers and percent yield.
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How to find the limiting reagent
The limiting reactant is the one that runs out first and so caps how much product can form. Convert each reactant's mass to moles (grams / molar mass), divide by its coefficient in the balanced equation, and the smallest result is the limiting reactant. The theoretical yield is that smallest value x the product coefficient x the product's molar mass. Percent yield = actual yield / theoretical yield x 100.
Worked example. 2 H2 + O2 → 2 H2O with 4.00 g H2 (2.016 g/mol) and 32.0 g O2 (31.998 g/mol). H2: 1.984 mol / 2 = 0.9921. O2: 1 mol / 1 = 1. Hydrogen is smaller, so H2 is limiting. Theoretical yield = 0.9921 x 2 x 18.015 = 35.74 g of water. If you collect 30.0 g, percent yield = 83.93%. Oxygen left over: 0.256 g.
The tool trusts your balanced equation. An unbalanced one gives a wrong answer without warning, and it assumes the reaction goes to completion with a single product pathway.
Frequently asked questions
What is a limiting reagent?
The reactant that is used up first, so it limits the amount of product. The other reactants are in excess and some is left over.
How do I find the limiting reagent?
Convert grams to moles, divide each by its balanced-equation coefficient, and pick the smallest value.
What is theoretical yield?
The most product the reaction could give, calculated from the limiting reactant. Actual yield is what you really collect.
Why is my percent yield over 100%?
Usually the product is wet, impure or weighed with leftover solvent, or an input is wrong. A true yield cannot exceed the theoretical one.
Sources
Checked
- OpenStax Chemistry 2e, 4.4 Reaction Yields (limiting reactant, theoretical and percent yield)
- OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept
- CIAAW: Abridged Standard Atomic Weights 2024
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.