Stoichiometry Calculator
Calculate reactant and product amounts from a balanced equation.
Enter the coefficient and chemical formula for each reactant/product. Fill in the mass (g) for one known species — all others will be calculated.
| Formula | Coeff. | Molar mass (g/mol) | Moles (mol) | Mass (g) |
|---|---|---|---|---|
| H2 | 2 | 2.016 | 0.625039 | 1.26008g |
| O2(known) | 1 | 31.998 | 0.31252 | 10g |
| H2O | 2 | 18.015 | 0.625039 | 11.2601g |
Learn more
What it does
The Stoichiometry Calculator uses a balanced equation to compute the moles and masses of every reactant and product from a single known mass. It is the core tool for quantitative chemistry, from lab prep to yield estimation. Real reactions are limited by whichever reactant runs out first. After balancing the equation, this tool converts a single known mass into the theoretical amounts of every other species.
How it works
Stoichiometry rests on the mole ratios given by the coefficients of a balanced equation. The amount ratio between any two species equals the ratio of their coefficients: n_A / n_B = coeff_A / coeff_B. The calculator takes one known mass m, converts it to moles with n = m / M, scales by the coefficient ratios to get every other species' moles, then converts back to mass with m = n x M. The result is theoretical: actual yield is lower because of side reactions, incomplete conversion and work-up losses. Percent yield = actual / theoretical × 100%. For limiting-reagent problems, compute each reactant's requirement and compare it against what you actually have.
Worked example
For 2 H2 + O2 -> 2 H2O, if you start with 4.032 g H2 (molar mass 2.016 g/mol, so 2.000 mol), the 2:1:2 ratio gives 1.000 mol O2 (32.00 g) and 2.000 mol H2O (36.03 g). Scaling backwards: to make 10.0 g of water you need 10 / 18.015 = 0.555 mol, which requires 0.555 mol H₂ (1.12 g) and 0.278 mol O₂ (8.89 g) — the same 2 : 1 : 2 ratios applied from product to reactants.
When to use it
Use it to find how much reactant you need for a target amount of product. Use it to predict the mass of product from a known starting mass. Use it to check limiting reagent intuition by comparing required versus available masses. Use it to plan how much starting material to weigh for a target product amount, to predict gas volumes in teaching labs, and to double-check reagent budgets before ordering.
FAQ
- What is stoichiometry in chemistry?
- Stoichiometry is the calculation of reactant and product amounts using the coefficients of a balanced equation, which give the exact mole ratios between substances.
- How do I calculate mass from a balanced equation?
- Convert your known mass to moles, multiply by the coefficient ratio to get moles of the target species, then multiply by its molar mass to get mass.
- Why must the equation be balanced first?
- Only a balanced equation conserves atoms, so its coefficients reflect the true mole ratios. An unbalanced equation gives wrong ratios and wrong masses.
- What is a limiting reagent?
- The limiting reagent is the reactant that runs out first and caps the amount of product; stoichiometry tells you how much of each other species is consumed or formed relative to it.