Excess Reagent Calculator

Calculator Inputs

Reaction Label

Reactant A Name

Reactant B Name

Product Name

Coefficient: Reactant A

Coefficient: Reactant B

Coefficient: Product

Reactant A Amount

Reactant B Amount

Unit: Reactant A

Unit: Reactant B

Molar Mass: Product (g/mol)

Molar Mass: Reactant A (g/mol)

Molar Mass: Reactant B (g/mol)

Purity: Reactant A (%)

Purity: Reactant B (%)

Use mass or mole inputs. Purity correction is applied before stoichiometric comparison.

Example Data Table

This example shows a common hydrogen and oxygen reaction using mass input values.

Reaction	Reactant A	Amount	Reactant B	Amount	Product
2H2 + O2 → 2H2O	Hydrogen	10 g	Oxygen	50 g	Water
N2 + 3H2 → 2NH3	Nitrogen	28 g	Hydrogen	10 g	Ammonia
CaCO3 + 2HCl → CaCl2 + CO2 + H2O	Calcium carbonate	25 g	Hydrochloric acid	30 g	Carbon dioxide

Formula Used

1. Convert each input to moles

moles = mass / molar mass for gram or kilogram input.

2. Correct for purity

effective moles = raw moles × purity / 100

3. Compare stoichiometric progress

extent = effective moles / stoichiometric coefficient

The smaller extent identifies the limiting reagent.

4. Find theoretical product yield

product moles = reaction extent × product coefficient

product mass = product moles × product molar mass

5. Find leftover excess reagent

leftover moles = effective moles − consumed moles

6. Estimate percent excess

percent excess = (actual excess / required stoichiometric amount) × 100

How to Use This Calculator

Enter the reaction label for your reference.
Provide names for two reactants and the main product.
Enter balanced stoichiometric coefficients from the chemical equation.
Input each reactant amount and choose its unit.
Add molar masses for any reactants entered by mass.
Set purity percentages when materials are not perfectly pure.
Click Calculate Excess Reagent to view results above the form.
Review limiting reagent, excess leftover, theoretical yield, conversions, and graph.

Frequently Asked Questions

1. What does excess reagent mean?

An excess reagent is the reactant supplied beyond the exact stoichiometric requirement. It remains partly unused after the limiting reagent is fully consumed in the reaction.

2. What is a limiting reagent?

The limiting reagent is the reactant that runs out first. It determines the maximum theoretical amount of product that can form under the selected reaction conditions.

3. Why do I need balanced coefficients?

Balanced coefficients define the correct mole ratio between reactants and products. Wrong coefficients produce incorrect limiting reagent selection, leftover values, and theoretical yield.

4. Can I enter mass instead of moles?

Yes. The calculator accepts grams, kilograms, millimoles, and moles. When mass units are selected, molar mass is used to convert your input into moles.

5. How does purity affect the answer?

Purity reduces the chemically active amount of a reactant. The calculator multiplies raw moles by purity percentage before comparing stoichiometric availability.

6. What does theoretical yield represent?

Theoretical yield is the maximum product amount predicted from stoichiometry, assuming complete conversion of the limiting reagent and no side reactions or process losses.

7. Why is leftover mass important?

Leftover mass helps with material planning, waste estimation, and safety review. It also supports cost calculations when one reactant is deliberately fed in excess.

8. Can I use this for laboratory and industrial reactions?

Yes, for stoichiometric estimation. However, real systems may also need side reactions, incomplete conversion, solvent effects, equilibrium, and process efficiency checks.