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Calculator Answers

Limiting Reactant Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A limiting reactant calculator is a stoichiometric tool that identifies the reagent in a chemical reaction that is fully consumed first, thereby limiting the amount of product formed. The calculator processes the balanced chemical equation, reactants' chemical formulas, and their initial masses or moles. It calculates the molar ratio of reactants and determines which substance restricts the theoretical yield of the reaction products, showing excess amounts remaining. Chemistry students and laboratory professionals use this tool to optimize reaction yields and plan chemical synthesis.

Enter the balanced equation's coefficients and the available amount of each of two reactants (in moles, or grams with molar mass). The calculator returns the limiting reactant, the excess, and an optional theoretical product yield.

Quick Answer

Identify the limiting reactant and calculate theoretical yield. Enter the reactants' quantities and chemical coefficients to solve stoichiometry problems.

Balanced equation coefficients

Enter the coefficients from a balanced equation and the available amount of each reactant. The calculator finds which one runs out first.

Reactant A

e.g. 2

Amount unit

Reactant B

e.g. 2

Amount unit

Optional: theoretical product

Provide the product coefficient (and optionally its molar mass) to also get the theoretical product yield.

e.g. 2

g/mol

You need a balanced equation first. Try the balancing equations calculator if you have not balanced it yet.

Result

Limiting reactant

Reactant B

Excess: Reactant A; 1 mol left over

A reactions possible2 mol / 2 = 1
B reactions possible0.5 mol / 1 = 0.5
Reactions that run0.5 (limited by B)
Theoretical product1 mol

The limiting reactant runs out before the other; the excess reactant has leftover material when the reaction stops. The theoretical product is the maximum possible from the limiting reactant on a balanced equation.

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Examples

2 mol H2, 0.5 mol O2 in 2 H2 + O2 → 2 H2O

O2 limits; 1 mol H2O

5 mol N2, 9 mol H2 in N2 + 3 H2 → 2 NH3

H2 limits; 6 mol NH3

10 g Al, 20 g HCl in 2 Al + 6 HCl → 2 AlCl3 + 3 H2

HCl limits

Equal moles, equal coefficients

Both run out together

How it works

The limiting reactant is the one that supports the smallest number of unit reactions. The number of unit reactions a reactant can support is its available moles divided by its balanced coefficient.

Step 1 · reactions(A) = moles(A) / coef(A)

Step 2 · reactions(B) = moles(B) / coef(B)

Step 3 · smaller value = limiting reactant

With grams: moles = grams / molar mass before comparing.

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Molality CalculatorSolve molality (mol/kg) = moles of solute / kg of solvent for any of the three variables, with an optional grams-to-moles helper. Includes a clear molality vs molarity reminder.

Frequently asked questions

The limiting reactant is the one that runs out first in a chemical reaction. It limits how much product can form. The other reactant is called the excess reactant; some of it remains unreacted when the limiting reactant is gone. 'Limiting reagent' is another name for the same concept.

Divide the moles of each reactant by its coefficient from the balanced equation. The smaller ratio is the limiting reactant. The calculator does this division for both reactants and reports the limiting and excess names plus the reactions actually possible.

The coefficient says how many moles of that reactant are consumed per 'unit' reaction. Dividing available moles by coefficient gives the number of unit reactions each reactant can support on its own; the smaller number wins.

Yes. Switch the amount unit to grams for either or both reactants and add the molar mass. The calculator converts to moles using n = m / M before comparing.

Yes. The whole approach uses the balanced coefficients. If the equation is not balanced, the limiting reactant calculation will be wrong. Use the balancing equations calculator first if needed.

Once you know how many unit reactions are possible (limited by the limiting reactant), multiply by the product's coefficient to get product moles. Multiply by the product's molar mass to get grams. The calculator returns both when the optional product fields are filled.