Stoichiometry Calculator
This stoichiometry calculator converts grams of one substance into grams of another using a balanced chemical equation: grams to moles, the mole ratio, then moles back to grams. Burning 10.000 g of methane gives 27.432 g of carbon dioxide.
Sources: IUPAC atomic weights 2023 Updated
What is Stoichiometry?
Stoichiometry is the calculation of quantities in chemical reactions. It allows you to determine how much product will form from given amounts of reactants, or how much reactant is needed to produce a desired amount of product. This calculator helps you perform these calculations quickly and accurately.
How to Use This Calculator
- Enter a balanced chemical equation using standard notation (e.g., 2H2 + O2 = 2H2O)
- Click "Parse Equation" to analyze the equation
- Select your known compound and enter its amount in grams
- Select the target compound you want to calculate
- Click "Calculate" to see the result
Enter Chemical Equation
Example equations you can try:
- • 2H2 + O2 = 2H2O (Water formation)
- • CH4 + 2O2 = CO2 + 2H2O (Methane combustion)
- • 2Al + 3Cl2 = 2AlCl3 (Aluminum chloride formation)
- • N2 + 3H2 = 2NH3 (Ammonia synthesis)
Understanding the Results
The calculator uses the following steps:
- Molar Mass Calculation: Determines the mass of one mole of each compound
- Mole Conversion: Converts the known mass to moles using molar mass
- Stoichiometric Ratio: Uses coefficients from the balanced equation to relate compounds
- Target Calculation: Converts moles of target compound back to grams
The three steps of a stoichiometry problem
Every mass-to-mass problem follows the same chain, as in OpenStax Chemistry: grams of A → moles of A → moles of B → grams of B.
Masses of different compounds cannot be compared directly, because each has its own molar mass. The coefficients of the balanced equation count particles, so they only apply once the masses are turned into moles.
How to convert grams to grams: burning methane
Enter CH4 + 2O2 = CO2 + 2H2O and press Parse Equation. Choose CH4 as the known compound with 10 grams, and CO2 as the target.
- Molar masses: CH₄ = 12.011 + 4 × 1.008 = 16.043 g/mol, and CO₂ = 12.011 + 2 × 15.999 = 44.009 g/mol.
- Grams to moles: 10.000 g ÷ 16.043 g/mol = 0.623325 mol of CH₄.
- Mole ratio: the equation has 1 CO₂ for every 1 CH₄, so 0.623325 mol × 1/1 = 0.623325 mol of CO₂.
- Moles to grams: 0.623325 mol × 44.009 g/mol = 27.432 g of CO₂.
The same 10 g of methane makes water at a 1 : 2 ratio: 1.246650 mol × 18.015 g/mol = 22.458 g of H₂O. It uses 1.246650 mol of O₂, which is 39.890 g.
Check that mass is conserved: 10.000 g + 39.890 g of reactants = 27.432 g + 22.458 g of products = 49.890 g. Adding several masses like this is what the mass calculator does.
Molar masses used
| Element | Atomic weight | Compound | Molar mass (g/mol) |
|---|---|---|---|
| H | 1.008 | CH₄ | 16.043 |
| C | 12.011 | O₂ | 31.998 |
| O | 15.999 | CO₂ | 44.009 |
| N | 14.007 | H₂O | 18.015 |
The atomic weights are the IUPAC standard values, rounded to three decimals.
From theoretical yield to percent yield
27.432 g is the theoretical yield, the most the reaction can make. If an experiment collects 25.0 g, the percent yield is 25.0 ÷ 27.432 × 100 = 91.1%, a part-of-a-whole sum the percentage calculator works out. How far the result falls short is a percent error of 8.9%.
What the calculator can read
The equation must already be balanced; the calculator does not check or balance it. Formulas are read as capital-letter element symbols, an optional lowercase letter and an optional count, so Ca, Cl2 and H2O work.
Brackets inside a formula, as in Ca(OH)2, and hydrate notation are not supported yet. Write such formulas out in full, for example CaO2H2.
Questions about stoichiometry
What is a mole ratio?
The ratio of the coefficients of two substances in a balanced equation. In CH₄ + 2O₂ → CO₂ + 2H₂O, the ratio of H₂O to CH₄ is 2 : 1, so every mole of methane burned gives two moles of water.
How do you work out a molar mass?
Add up the atomic weights of the atoms in the formula. For CO₂: 12.011 + 2 × 15.999 = 44.009 g/mol.
Does the calculator balance equations?
No. It reads the coefficients exactly as typed, so the equation has to be balanced first. An unbalanced equation still gives a number, but the mole ratio, and so the answer, will be wrong.
Sources
- IUPAC Commission on Isotopic Abundances and Atomic Weights: Atomic Weights of the Elements 2023 — standard atomic weights used for the molar masses, rounded to 3 decimal places (checked October 12, 2026)
- OpenStax Chemistry 2e, 4.3 Reaction Stoichiometry — converting between masses of reactants and products through moles and the balanced equation (checked October 16, 2026)