Limiting Reactant CalculatorBalance the equation, find the limiting reagent and see what is left over
Type any reaction. The calculator balances it, compares every reactant by moles ÷ coefficient, and reports the maximum product and the excess remaining.
By Manzoor Ahmad, web development expert · Updated How do you find the limiting reactant?
Convert each reactant to moles and divide by its coefficient in the balanced equation; the smallest result is the limiting reactant. For N₂ + 3H₂ → 2NH₃ with 28.0 g N₂ (1.00 mol) and 10.0 g H₂ (4.96 mol): 1.00 ÷ 1 = 1.00 and 4.96 ÷ 3 = 1.65, so nitrogen is limiting. It can make 2.00 mol (34.0 g) of ammonia, leaving 3.96 g of hydrogen unused.
Reactant amounts
Moles ÷ coefficient (shortest bar runs out first)
Products (theoretical yield)
| Product | Moles | Mass |
|---|
Reactants used and left over
| Reactant | Start | Used | Left over |
|---|
The limiting reactant method
- Balance the equation. Coefficients are the mole ratios.
- Convert every reactant to moles using its molar mass.
- Divide each by its coefficient. The smallest value is the extent of reaction, ξ, and belongs to the limiting reactant.
- Product moles = coefficient × ξ. Multiply by molar mass for grams.
- Excess left = start − coefficient × ξ for every other reactant.
The sandwich analogy
Worked examples
1. Ammonia synthesis (Haber process)
28.0 g N₂ = 0.9995 mol; 10.0 g H₂ = 4.960 mol. Ratios: 0.9995 ÷ 1 = 0.9995 and 4.960 ÷ 3 = 1.653. Nitrogen limits. NH₃ = 2 × 0.9995 = 1.999 mol = 34.04 g. Hydrogen used = 3 × 0.9995 × 2.016 = 6.045 g, so 3.955 g H₂ is left.
2. Aspirin with excess acetic anhydride
C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + C₂H₄O₂. With 2.00 g salicylic acid (0.01448 mol) and 5.0 mL acetic anhydride (about 5.4 g, 0.0529 mol), salicylic acid is limiting, giving a theoretical 2.609 g of aspirin. Labs deliberately use excess anhydride so the more valuable salicylic acid is fully used.
Limiting reactant FAQs
The limiting reactant (or limiting reagent) is the reactant that runs out first in a chemical reaction. It sets the maximum amount of product that can form; the other reactants are left over in excess.
Convert each reactant to moles, then divide each by its coefficient in the balanced equation. The reactant with the smallest value is limiting. For N2 + 3H2 → 2NH3 with 1.00 mol N2 and 4.96 mol H2: 1.00 ÷ 1 = 1.00 and 4.96 ÷ 3 = 1.65, so N2 is limiting.
Because reactants combine by moles, not grams, and in the ratio set by the equation's coefficients. 10 g of hydrogen is far more moles than 28 g of nitrogen, and the 3 : 1 ratio also matters.
Yes. The coefficients give the mole ratios, so an unbalanced equation gives wrong answers. This calculator balances the equation for you if the coefficients are missing or incorrect.
Multiply the limiting reactant's 'moles ÷ coefficient' value by the excess reactant's coefficient to get the moles used, subtract that from the moles you started with, and convert back to grams.
The extent of reaction (ξ) is the smallest 'moles ÷ coefficient' value. Every species changes by its coefficient × ξ, so product moles = coefficient × ξ.
Type reactants and products separated by an arrow, such as N2 + H2 -> NH3. You can use ->, →, = or =>. State symbols like (g) or (aq) are ignored.
If two reactants give exactly the same 'moles ÷ coefficient' value, they are present in the exact stoichiometric ratio and run out together. The calculator reports that case.
Check the formulas for typos and capitalization. Some equations, such as certain redox reactions written without electrons or ions, cannot be balanced from neutral formulas alone.
No. Theoretical yield is the maximum product from the limiting reactant. Actual yield is what you collect, which is usually lower. Their ratio is the percent yield.
Manufacturers deliberately put a cheaper reactant in excess so that the expensive one is limiting and fully used. Calculating the limiting reagent is how batch quantities are planned.
Enter neutral formulas. Charges are not parsed, so for ionic equations use full compounds, for example AgNO3 + NaCl -> AgCl + NaNO3.
Sources
This limiting reactant calculator assumes the reaction goes to completion as written. Real reactions may stop at equilibrium or form side products.