Stoichiometry Calculator
Enter a mole ratio from your balanced equation, plus known grams or moles of one substance, to find the amount of another.
Enter the mole ratio between A and B from your own balanced equation (e.g. 2 H₂ + O₂ → 2 H₂O has a 2:2 ratio of H₂ to H₂O), plus the known amount of A.
coefficient of A in the balanced equation
coefficient of B in the balanced equation
to convert grams to moles
to also get grams of B, not just moles
4 g ÷ 2.016 g/mol = 1.9841 mol of A. mol B = mol A × (ratio B / ratio A) = 1.9841 × (3/2) = 2.9762 mol.
Mass of B = 2.9762 mol × 18.015 g/mol = 53.616 g.
Mole-Ratio Stoichiometry
A balanced chemical equation is a recipe in moles: the coefficients tell you exactly how many moles of each substance react or form relative to each other. Once you know the mole ratio between two substances (A and B) and how much of A you have, you can scale directly to how much B reacts or forms.
Working in grams instead of moles just adds one conversion step on each end: divide a known mass by its molar mass to get moles, do the mole-ratio math, then multiply by the other substance's molar mass to get its mass.
Worked example
In 2 H2 + O2 → 2 H2O, how many grams of water form from 10 g of H2 (assume plenty of O2)?
10 g H2 ÷ 2.016 g/mol = 10.0000 mol H2. Mole ratio of H2 to H2O is 2:2 (1:1), so mol H2O = 10.0000 × (2/2) = 10.0000 mol.
Mass of H2O = 10.0000 mol × 18.015 g/mol = 180.150 g.
Common mistake
Using the mass ratio instead of the mole ratio -- stoichiometry coefficients are always mole counts, never mass. Skipping the mass-to-moles conversion and multiplying grams directly by the coefficients gives a wrong answer.
Keep going
- Stoichiometry gives the theoretical amount of product a reaction should produce; the percent yield calculator compares that theoretical number to what you actually recovered. Percent Yield Calculator
- Converting between grams and moles of each substance in a stoichiometry problem needs each compound's molar mass, which this calculator's parser computes from its formula. Molar Mass Calculator
Frequently Asked Questions
What is a mole ratio?
The ratio between the coefficients of two substances in a balanced chemical equation. In 2 H2 + O2 → 2 H2O, the mole ratio of H2 to H2O is 2:2 (or 1:1), meaning every 2 moles of H2 that react produce 2 moles of H2O.
Why does this calculator ask for a ratio instead of a full equation?
Balancing an arbitrary chemical equation automatically is a much bigger problem than the mole-ratio math itself. If you already have your balanced equation (from a textbook problem or your own work), you already know the two coefficients you need -- this calculator picks up from there.
Do I need molar masses to use this calculator?
Only if you're starting from grams instead of moles, or want the answer in grams instead of moles. If you already have moles of the known substance and only need moles of the other, you can skip both formula fields.
What's the difference between stoichiometry and percent yield?
Stoichiometry predicts how much product a reaction should make in theory (theoretical yield); percent yield compares that prediction to what you actually recovered in the lab. Run stoichiometry first, then use its result as the theoretical yield in the percent yield calculator.
What if I have two known reactants, not just one?
That's a limiting-reactant problem: run this calculator once for each reactant against the same product, and whichever gives the smaller predicted amount of product is the actual, limiting answer -- the other reactant is in excess.
Related Calculators
- Percent Yield Calculator — Reactions & Stoichiometry
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- Dilution Calculator — Solutions & Concentration