Chemistry overview
What is a limiting reactant?
A limiting reactant is consumed first when reactants combine according to a balanced chemical equation. Once that reactant is exhausted, additional product cannot form.
The other substance is the excess reactant. Some of it remains after the maximum theoretical reaction has occurred.
Formula
Limiting-reactant formula
Reaction capacity= available moles ÷ balanced coefficient
- Available moles = mass ÷ molar mass when the input uses grams.
- The smaller reaction capacity identifies the limiting reactant.
- Product moles = reaction capacity × product coefficient.
- Excess moles = available moles − moles consumed.
Calculation method
How to find the limiting reactant
- Balance the chemical equation.
- Record the available amount of each reactant.
- Convert every mass input to moles.
- Divide each reactant's moles by its coefficient.
- Select the smaller reaction capacity.
- Use that capacity to calculate product and excess reactant.
Worked example
Hydrogen and oxygen example
For 2H₂ + O₂ → 2H₂O, suppose the reaction starts with 2 mol H₂ and 2 mol O₂.
- Hydrogen capacity: 2 ÷ 2 = 1
- Oxygen capacity: 2 ÷ 1 = 2
- Hydrogen has the smaller capacity, so it is the limiting reactant.
- Product: 1 × 2 = 2 mol H₂O
- Oxygen remaining: 2 − 1 = 1 mol O₂
Mass inputs
Using grams in the calculator
A balanced equation compares particles and moles rather than grams directly. The calculator therefore converts every mass input to moles before comparing reaction capacities.
Enter the molar mass in grams per mole whenever a reactant or product uses grams. Use the Molecular Weight Calculator when you need to determine a compound's molar mass.
Balanced coefficients
Why coefficients matter
Coefficients state the mole ratio in a balanced equation. Comparing raw mole amounts without dividing by coefficients can identify the wrong limiting reactant.
In N₂ + 3H₂ → 2NH₃, hydrogen moles must be divided by three, while nitrogen moles are divided by one.
Common errors
Common limiting-reactant mistakes
- Using coefficients from an unbalanced equation.
- Comparing grams directly instead of converting them to moles.
- Comparing mole amounts without normalizing by coefficients.
- Entering an incorrect molar mass.
- Treating theoretical yield as actual experimental yield.
Calculation limits
Assumptions and limitations
The chemical equation must be correctly balanced, and all amounts, coefficients, units, and molar masses must correspond to the selected substances.
The calculation assumes pure reactants, complete reaction, and the stated stoichiometric pathway.
The result does not account for impurities, equilibrium, side reactions, incomplete conversion, measurement uncertainty, or experimental product loss.
Supporting tools
Continue the reaction calculation
Review single-substance mole ratios with the Stoichiometry Calculator, or convert between mass and moles with the Mass to Moles Calculator.
Questions and answers
Limiting reactant calculator FAQ
What is a limiting reactant?
The limiting reactant is the substance that runs out first according to the balanced-equation coefficients and therefore limits the amount of product that can form.
How does the calculator identify the limiting reactant?
It converts each available amount to moles, divides each mole quantity by its balanced coefficient, and selects the smaller reaction capacity.
Can I enter reactants in grams?
Yes. Select grams and enter the correct molar mass for each reactant expressed in grams.
What does co-limiting mean?
Reactants are co-limiting when their coefficient-normalized reaction capacities are equal, so neither reactant remains in excess.
Does the result equal the actual laboratory yield?
No. The calculated product is a theoretical yield that assumes complete reaction, pure reactants, and no experimental loss.