# Limiting reagent

The limiting reagent is the reactant that runs out first and so decides how much product forms. The other reactants are in excess. To find it, compare the moles of each reactant with the balanced ratio.

Chemistry (university) · Part 04: Stoichiometry and reactions in solution · Card 4 of 9 · Last updated 2026-10-19

## Analogy

Making sandwiches: with 10 slices of bread and 4 slices of cheese, cheese limits you to 4 sandwiches.

## Example

**2.0 mol N₂ + 3.0 mol H₂**

N₂ + 3H₂ → 2NH₃
N₂ needs 6.0 mol H₂; only 3.0 present
H₂ is limiting
Product: 3.0 × (2/3) = 2.0 mol NH₃

## In practice

A factory orders extra of the cheap reactant to make sure the expensive one is used up completely.

## Practice

### Flashcards

- **Which reactant is the limiting reagent?** The one that is used up first and so limits the product.

### Interview questions

**Explain how to identify the limiting reagent and why it matters.**

- Convert masses to moles
- Divide by the coefficients
- The smallest result limits
- Product amount is set by the limiting reagent

## Questions people ask

### What is the quickest way to find the limiting reagent?

Divide the moles of each reactant by its coefficient in the balanced equation. The reactant with the smallest result is limiting. This avoids working out product amounts for each reactant, though that method also works. Remember to convert any masses into moles first.

### What happens to the excess reagent?

The excess reagent is left over after the reaction. You can calculate how much remains by working out how much was used up. Subtract that from the starting amount. It stays in the flask mixed with the products and often has to be separated.

## Related concepts

- [Percent yield](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-4/chemistry-university-part-4-concept-5)
- [Mole ratio from a balanced equation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-4/chemistry-university-part-4-concept-3)

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