Stoichiometry Calculator

Convert between moles of reactants and products using stoichiometric ratios.

Enter known moles and coefficients for Species A. Enter coefficient for Species B to find its moles.

How It Works

Enter the molar amounts and stoichiometric coefficients for two species in a balanced reaction. The calculator uses the mole ratio (coefficient B ÷ coefficient A) to find how many moles of species B are produced or consumed per moles of species A. The mole ratio comes directly from the balanced chemical equation.

Formula

moles B = moles A × (coefficient B / coefficient A)

Worked Example: Combustion of Hydrogen

Balanced equation: 2 H₂ + O₂ → 2 H₂O

  1. Coefficient of H₂ = 2, coefficient of O₂ = 1
  2. If you have 4 mol H₂, moles of O₂ needed = 4 × (1/2) = 2 mol
  3. Moles of H₂O produced = 4 × (2/2) = 4 mol

Worked Example: Haber Process (Ammonia Synthesis)

Balanced equation: N₂ + 3 H₂ → 2 NH₃

  1. Starting with 5 mol N₂: H₂ needed = 5 × (3/1) = 15 mol
  2. NH₃ produced = 5 × (2/1) = 10 mol

The Full Stoichiometry Roadmap

Most stoichiometry problems follow four steps: (1) Start with what you know in any unit (grams, liters of gas, particles). (2) Convert to moles using molar mass, molar volume, or Avogadro's number. (3) Apply the mole ratio from the balanced equation. (4) Convert moles of the desired species back to whatever unit the question asks for. This calculator handles step 3; pair it with the Moles to Grams Calculator for a complete workflow.

Limiting Reagents and Theoretical Yield

When you have two or more reactants, you need to identify the limiting reagent — the substance that runs out first and determines the maximum amount of product. Run the stoichiometry calculation for each reactant separately; whichever predicts the smallest amount of product is the limiting reagent. See the Limiting Reagent Calculator for an automated comparison.

Frequently Asked Questions

What is stoichiometry?

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It is based on the law of conservation of mass and the mole ratios from balanced equations.

How do you use stoichiometry?

Convert known quantities to moles using molar mass (or volume and pressure for gases). Use the mole ratio from the balanced equation to find moles of the target species. Then convert back to grams, liters, or particles as needed.

Why do we need a balanced equation?

Coefficients in a balanced equation give the exact mole ratios in which substances react and are produced. An unbalanced equation violates conservation of mass and gives wrong ratios, leading to incorrect calculations.

Can stoichiometry be applied to gases?

Yes. At standard temperature and pressure (STP), 1 mole of any ideal gas occupies 22.4 L. Use PV = nRT for other conditions. Once you have moles, the mole ratio step is identical to any other stoichiometry problem.

What is the difference between theoretical yield and actual yield?

Theoretical yield is the maximum product predicted by stoichiometry assuming 100% efficient, complete reaction. Actual yield is what you obtain in practice. Percent yield = (actual / theoretical) × 100%.