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Limiting Reactant Calculator

Find the limiting reactant in a balanced chemical reaction using stoichiometric coefficients and reactant amounts. The calculator also estimates the excess reactant and theoretical product amount in moles.

Stoichiometry Balanced Coefficients Instant Results

Limiting Reactant Calculator

Enter the coefficients from your balanced equation and the available moles of each reactant.

Method: The calculator compares available moles ÷ stoichiometric coefficient for each reactant. The smaller value identifies the limiting reactant.
Balanced Reaction Setup
A
→
A
Reactant A
B
Reactant B
Limiting Reactant Test
Reaction capacity = Available Moles ÷ Stoichiometric Coefficient. The reactant with the smaller capacity is limiting.
Limiting Reactant
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Reaction Extent -
Product Moles -
Reactant A Left -
Reactant B Left -
Chemistry Basics

What Is a Limiting Reactant?

The limiting reactant is the reactant that is consumed first according to the stoichiometric ratios in a balanced chemical equation. Once it is used up, additional product cannot be formed through that reaction unless more of the limiting reactant becomes available.

Other reactants may remain after the limiting reactant has been consumed. These are commonly called excess reactants.

Main Formula

Limiting Reactant Formula

For two reactants A and B in a balanced equation, compare the available number of moles divided by each reactant's stoichiometric coefficient.

Reaction Capacity = Moles ÷ Coefficient Compare nA ÷ a with nB ÷ b. The smaller value determines the limiting reactant.
How It Works

How to Use the Limiting Reactant Calculator

01

Use a Balanced Equation

Enter the stoichiometric coefficients for both reactants and the product from your balanced chemical equation.

02

Enter Available Moles

Enter the amount of each reactant in moles. If your data is given as mass, convert it to moles before using this calculator.

03

Compare Reaction Capacity

The calculator divides each mole amount by its coefficient and identifies the smaller value.

Worked Example

Limiting Reactant Example

Consider the balanced reaction: 2H₂ + O₂ → 2H₂O. Suppose 4 moles of H₂ and 3 moles of O₂ are available.

Reactant Available Moles Coefficient Moles ÷ Coefficient
H₂ 4 mol 2 2
O₂ 3 mol 1 3

Because 2 is smaller than 3, H₂ is the limiting reactant. The reaction can proceed through 2 stoichiometric reaction units, producing 4 moles of H₂O according to the balanced equation.

Stoichiometry

How to Calculate the Excess Reactant

Once the limiting reactant is identified, the reaction extent tells you how much of each reactant can be consumed. Multiply the reaction extent by each stoichiometric coefficient.

Reactant Left = Initial Moles − Consumed Moles Consumed moles = reaction extent × reactant coefficient.
Product Calculation

Theoretical Product Moles

For the simplified reaction used by this calculator, theoretical product moles are calculated from the maximum reaction extent and the product's stoichiometric coefficient.

Product Moles = Reaction Extent × Product Coefficient This represents the stoichiometric theoretical amount, assuming the reaction proceeds according to the balanced equation.
Important Information

Why the Balanced Equation Matters

The limiting reactant cannot be determined correctly from reactant amounts alone. The coefficients in the balanced equation define the mole ratio required by the reaction.

Educational Note: This calculator assumes a simplified reaction containing two reactants and one product. Enter values from an already balanced equation. It is designed for stoichiometry calculations and classroom-style chemistry problems.
Frequently Asked Questions

Limiting Reactant Calculator FAQs

What is a limiting reactant?
A limiting reactant is the reactant that restricts how much product can be formed according to the stoichiometric ratios in the balanced equation.
How do you find the limiting reactant?
For each reactant, divide the available moles by its coefficient in the balanced equation. The reactant producing the smaller value is limiting.
What is an excess reactant?
An excess reactant is present in more than the stoichiometric amount required to react with the limiting reactant, so some can remain after the limiting reactant is consumed.
Why must the chemical equation be balanced?
A balanced equation provides the correct stoichiometric coefficients and therefore the correct mole ratios between reactants and products.
Can I enter grams directly?
This version uses moles directly. When a problem gives mass, first convert each mass to moles using the substance's molar mass.
What if both reactants give the same ratio?
If both available-moles-to-coefficient ratios are equal, the reactants are present in the exact stoichiometric proportion for this simplified calculation.
How is theoretical product calculated?
The maximum reaction extent is multiplied by the product coefficient from the balanced equation. This gives theoretical product moles for the simplified reaction model.
Does a limiting reactant calculator predict actual yield?
No. Stoichiometry gives a theoretical amount based on the balanced equation. Actual experimental yield can differ from the theoretical value.