Stoichiometry Calculator
Enter a reaction and how much of each reactant you have. The calculator balances it, finds the limiting reactant and gives the theoretical yield.
Balanced: N2 + 3H2 → 2NH3
| Species | Molar mass | Start (mol) | mol ÷ coeff. | Used / formed (mol) | Left / made (g) |
|---|---|---|---|---|---|
| N2 (limiting) | 28.014 | 0.9995 | 0.9995 | 0.9995 | 0 left |
| H2 | 2.016 | 4.9603 | 1.6534 | 2.9985 | 3.955 left |
| NH3 | 17.031 | 0 | — | 1.999 | 34.045 made |
- N2: 0.9995 mol ÷ 1 = 0.9995
- H2: 4.9603 mol ÷ 3 = 1.6534
- Smallest value → N2 limits the reaction (extent 0.9995 mol).
- NH3: 0.9995 × 2 = 1.999 mol × 17.031 g/mol = 34.045 g
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AI answers can be wrong. The calculators on this page are deterministic; trust them for numbers.
The stoichiometry road map
grams A → moles A → moles B → grams B. Divide by molar mass, multiply by the mole ratio (coefficient of B ÷ coefficient of A), multiply by molar mass of B. With two reactants, do the first two steps for each and pick the one that makes less product; that is the limiting reactant.
Example: 28 g N₂ and 10 g H₂. N₂: 28 ÷ 28.014 = 1.000 mol, ÷ 1 = 1.000. H₂: 10 ÷ 2.016 = 4.96 mol, ÷ 3 = 1.65. Nitrogen is limiting, so NH₃ = 2 × 1.000 = 2.000 mol = 34.06 g.
Need just the ratio of actual to theoretical? Use the percent yield calculator.
Questions students ask
What is stoichiometry?
The arithmetic of chemical reactions: using the mole ratios from a balanced equation to get from an amount of one substance to an amount of another. The route is always grams → moles → mole ratio → moles → grams.
How do you find the limiting reactant?
Convert each reactant to moles, then divide by its coefficient in the balanced equation. The smallest result is the limiting reactant: it runs out first and sets how much product can form. The others are in excess.
What is theoretical yield?
The maximum mass of product the limiting reactant can make, assuming the reaction goes to completion with no losses. Actual yield ÷ theoretical yield × 100 gives percent yield.
Do I have to balance the equation first?
Yes, but this calculator does it for you. The mole ratios only mean something once the equation is balanced.