Stoichiometry Calculator
Balance a reaction, then scale every species by its mole ratio.
Enter a balanceable equation with an = or →, e.g. N2 + 3H2 = 2NH3.
Stoichiometry uses a balanced equation’s coefficients as mole ratios: moles of B = moles of A × (coefficient B ÷ coefficient A). For 2 H₂ + O₂ → 2 H₂O, 4.0 mol of H₂ makes 4.0 mol of H₂O (a 1:1 ratio), which is 4.0 × 18.015 = 72.06 g of water.
Mole ratios do the work
A balanced equation’s coefficients are mole ratios. Once you know the amount of one species, every other amount follows by multiplying through those ratios — then converting moles to grams with each molar mass.
grams ↔ moles via moles = grams ÷ molar mass
Worked example
For 2 H₂ + O₂ → 2 H₂O, how many grams of water form from 4.0 mol of H₂?
- 1 Read the mole ratio. H₂ to H₂O is 2 : 2, which simplifies to 1 : 1.
- 2 Scale to the unknown. moles H₂O = 4.0 mol H₂ × (2 ÷ 2) = 4.0 mol H₂O.
- 3 Convert moles to grams. mass = 4.0 mol × 18.015 g/mol = 72.06 g of water.
The grams-to-grams road map
The standard four-step path; this tool runs it for every species at once.
| Step | Operation | Tool used |
|---|---|---|
| 1. grams of known → moles | ÷ molar mass of known | Molar Mass |
| 2. moles known → moles unknown | × coefficient ratio | Balanced equation |
| 3. moles unknown → grams | × molar mass of unknown | Molar Mass |
| 4. (optional) → percent yield | actual ÷ theoretical × 100 | — |
Assumptions and pitfalls
The equation must be balanced first. Mole ratios come from the coefficients, so an unbalanced equation gives wrong ratios. This tool balances automatically before scaling.
This assumes one limiting reactant. Scaling from a single known amount presumes that species runs out first. To find the limiting reactant, compute how much product each reactant would make and take the smallest.
Common mistakes. Using subscripts instead of coefficients for the ratio, skipping the grams→moles conversion, and mixing up which molar mass belongs to which species.