This stoichiometry calculator does the classic grams → moles → mole ratio → moles → grams conversion: given the mass of one substance in a reaction, it finds the mass of any other substance the balanced equation connects it to.
What Is Stoichiometry?
Stoichiometry is the accounting of chemical reactions: the balanced equation’s coefficients fix the proportions in which substances react and form, and stoichiometry converts those proportions into grams you can weigh. Every industrial process — fertilizer plants, pharmaceuticals, rocket propellant — runs on these conversions. This calculator is part of our Chemistry Calculators collection; molar masses for any formula come from our Molar Mass Calculator.
The Roadmap (Never Skip the Moles)
grams A → (÷ molar mass A) → moles A → (× coefficient ratio) → moles B → (× molar mass B) → grams B
The coefficients in the equation relate moles, never grams directly — converting through moles is the whole trick, and skipping it is the most common homework error in chemistry.
Worked Example (Verified)
Ammonia synthesis: N₂ + 3H₂ → 2NH₃. Starting with 28.0 g of N₂ (molar mass 28.014 g/mol):
- Moles of N₂: 28.0 ÷ 28.014 ≈ 1.00 mol
- Mole ratio NH₃:N₂ = 2:1 → 2.00 mol NH₃
- Mass of NH₃: 2.00 × 17.031 = 34.06 g of ammonia — the theoretical yield
- (The same reaction needs 3 mol H₂ = 6.05 g of hydrogen — set A = N₂, B = H₂ with its molar mass 2.016 to check.)
What the reaction actually delivers versus this theoretical number is the job of our Percent Yield Calculator.
Limiting Reactants — the Next Idea
Real mixtures rarely match the equation’s proportions exactly. The reactant that runs out first — the limiting reactant — caps the product. The method: run this conversion from each reactant to the same product; the smaller product mass wins, and the other reactant is in excess. (Haber-process plants deliberately feed excess N₂ and recycle it.)
Related Tools
- Molar Mass Calculator — the g/mol inputs
- Percent Yield Calculator — theoretical vs actual
- Chemistry Calculators — the full collection
Frequently Asked Questions
What are the steps of a stoichiometry problem?
Balance the equation, convert the known mass to moles, apply the coefficient ratio to get moles of the target, then convert back to mass with the target’s molar mass.
Why can’t I use the coefficients with grams directly?
Because coefficients count particles (moles), and a mole of a heavy substance weighs more than a mole of a light one. 2 mol of NH₃ is 34 g; 2 mol of N₂ is 56 g — same count, different mass.
What is a mole ratio?
The ratio of two coefficients in the balanced equation — for N₂ + 3H₂ → 2NH₃, the ratio NH₃:H₂ is 2:3.
How do I find the limiting reactant?
Compute the product each reactant could make; the reactant producing the least product is limiting.
Does this calculator balance equations?
No — you enter the coefficients from your balanced equation. Balancing comes first; this tool does the conversions that follow.
Is this stoichiometry calculator free?
Yes — free, instant, no sign-up.