Stoichiometry Calculator

Convert between mass, moles, and molecules across a balanced chemical equation. Calculate theoretical yield, molar ratios, and gas volumes at STP.

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Moles of Target
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Moles of Known —
Mass of Target —
Molecules of Target —
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Mass of Target (g)
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Moles of Target —
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Moles of Target
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Moles of Known —
Theoretical Yield (g) —
Percent Yield —
Molecules of Target —
Gas Volume at T,P (L) —

How to Use This Calculator

  1. Enter the molar mass of your known species and its amount (g, mol, or molecules).
  2. Enter the stoichiometric ratio (coefficient of target ÷ coefficient of known).
  3. Enter the molar mass of the target species.
  4. Select the desired output unit.

Formula

mol_known = mass / MM_known

mol_target = mol_known × (coeff_target / coeff_known)

mass_target = mol_target × MM_target

Example

Example: 2H₂ + O₂ → 2H₂O. Starting with 4 g H₂ (MM = 2.016 g/mol): mol H₂ = 1.984. Ratio H₂O:H₂ = 1. mol H₂O = 1.984. Mass H₂O = 1.984 × 18.015 = 35.73 g.

Frequently Asked Questions

  • Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction, based on the law of conservation of mass. It uses the mole ratios from a balanced chemical equation to calculate how much of each substance is consumed or produced. For example, 2H₂ + O₂ → 2H₂O tells us that 2 mol of hydrogen gas reacts with 1 mol of oxygen gas to produce 2 mol of water. If you start with 4 mol H₂, stoichiometry predicts you will need 2 mol O₂ and produce 4 mol H₂O. Real reactions may produce less due to side reactions, incomplete reactions, or losses.
  • Moles = Mass (g) ÷ Molar mass (g/mol). The molar mass is found by summing the atomic masses of all atoms in the formula. For water (H₂O): molar mass = 2 × 1.008 + 15.999 = 18.015 g/mol. So 36.03 g ÷ 18.015 = 2.00 mol. For NaCl: molar mass = 22.990 + 35.453 = 58.44 g/mol. So 117 g ÷ 58.44 = 2.00 mol. This step — grams to moles — is always the first step in a stoichiometry calculation. Enter the molar mass and amount in this calculator to perform the conversion automatically.
  • STP (Standard Temperature and Pressure) is defined as 0°C (273.15 K) and 1 atm (101.325 kPa). At STP, exactly 1 mole of any ideal gas occupies 22.414 L (the molar volume). This allows quick gas volume calculations: moles × 22.4 = volume in liters at STP. SATP (Standard Ambient Temperature and Pressure) is defined as 25°C and 100 kPa; at SATP, 1 mole occupies 24.79 L. These values are used for gas stoichiometry: if 2 mol of H₂ is produced at STP, the volume is 2 × 22.4 = 44.8 L. The calculator includes an STP gas volume output when solving for gas products.
  • The mole ratio is taken directly from the stoichiometric coefficients of the balanced chemical equation. For the reaction 2H₂ + O₂ → 2H₂O, the mole ratios are: H₂:O₂ = 2:1, H₂:H₂O = 2:2 = 1:1, O₂:H₂O = 1:2. To use the mole ratio: multiply the moles of the known substance by (moles of target ÷ moles of known) from the balanced equation. In this calculator, enter the coefficient of the known species and the coefficient of the target species; the calculator applies the ratio automatically.
  • Percent yield = (Actual yield ÷ Theoretical yield) × 100%. The theoretical yield is the maximum possible amount of product calculated by stoichiometry, assuming the reaction goes to completion and no product is lost. The actual yield is what you actually collect in the lab. For example, if stoichiometry predicts 50 g of product but only 42 g is obtained, percent yield = (42 ÷ 50) × 100% = 84%. Yields below 100% occur due to side reactions, incomplete reactions, purification losses, and measurement error. Yields above 100% indicate impurities or measurement errors in the product.

Related Calculators

⚗️ Molar Mass Calculator 🥛 Limiting Reactant Calculator 📊 Percent Yield Calculator ⚛️ Empirical Formula Calculator
Sources & References (5) ▾
  1. Stoichiometry — ACS Chemistry for Life — American Chemical Society
  2. IUPAC Atomic Weights of the Elements 2021 — IUPAC
  3. NIST Chemistry WebBook — Stoichiometry Reference — NIST
  4. OpenStax Chemistry 2e, Chapter 4 — Stoichiometry of Chemical Reactions — OpenStax
  5. LibreTexts Chemistry — Stoichiometry — LibreTexts