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Stoichiometry

General

Stoichiometry

The calculation of the quantitative relationships between reactants and products in a balanced chemical equation, used to predict yields and required amounts.

Definition

Stoichiometry is the branch of chemistry concerned with the quantitative relationships between reactants and products in a chemical reaction, based on the mole ratios given by a balanced equation. It answers practical questions such as how many grams of a reactant are needed to produce a target amount of product, or which reactant will run out first when several are combined.

Every stoichiometric calculation follows the same basic path: convert a known mass or volume into moles, use the balanced equation's coefficients to find the equivalent moles of another substance in the reaction, then convert back into whatever unit (grams, liters, particles) the question asks for. This mole-to-mole bridge, provided directly by the coefficients of a balanced equation, is the foundation of all stoichiometric reasoning.

Stoichiometry underlies related calculations such as determining a compound's empirical formula from experimental composition data, and calculating atom economy to evaluate how efficiently a reaction converts starting materials into the desired product. The Empirical Formula Calculator and Atom Economy Calculator both apply stoichiometric mole-ratio logic to solve these problems.

Formula

Moles of B = Moles of A × (coefficient of B / coefficient of A)

Applied to convert between masses of two substances A and B in a balanced reaction:

Mass of B = (Mass of A / Molar Mass of A) × (coefficient of B / coefficient of A) × Molar Mass of B

Worked Example

Consider the balanced equation: 2H2 + O2 → 2H2O

How many grams of water form from 10 grams of hydrogen gas (molar mass 2.02 g/mol), assuming excess oxygen?

Moles of H2 = 10 g / 2.02 g/mol = 4.95 mol

Moles of H2O = moles of H2 × (2/2) = 4.95 mol (1:1 ratio from the equation)

Mass of H2O = 4.95 mol × 18.02 g/mol = 89.2 grams

So 10 grams of hydrogen gas, reacting completely with excess oxygen, produces about 89.2 grams of water. Apply the same mole-ratio approach to your own reactions using the tools above.

Key Things to Know

  • Everything routes through moles: Stoichiometry always converts given quantities into moles first, since the balanced equation's coefficients describe mole ratios, not mass ratios.
  • The limiting reactant caps the yield: When multiple reactants are combined, the one that produces the smallest amount of product when divided by its coefficient determines the maximum possible product, regardless of how much of the other reactants remain.
  • Empirical formula determination is applied stoichiometry: Working out a compound's simplest atomic ratio from percent composition data uses the exact same mole-conversion logic as reaction stoichiometry.
  • Theoretical yield is a ceiling, not a guarantee: Real reactions rarely achieve 100% of their calculated theoretical yield due to side reactions, incomplete conversion, and losses during product isolation.
  • Balanced equations are non-negotiable: Every stoichiometric calculation depends on starting from a correctly balanced chemical equation, since incorrect coefficients produce incorrect mole ratios and therefore incorrect answers.

Related Calculators

Empirical Formula Calculator →Calculate the empirical formula of a compound from elemental percentages or gram amounts. Find whole-number mole ratios for up to 5 elements.
Atom Economy Calculator →Calculate atom economy (AE) and atom efficiency of chemical reactions. Key green chemistry metric for measuring how much of the reactant mass ends up in the desired product.

Related Terms

MoleMole (SI unit of amount of substance)Empirical FormulaEmpirical Formula

Frequently Asked Questions

Stoichiometry is used to calculate exactly how much of each reactant is needed, or how much product will form, based on the mole ratios given by a balanced chemical equation. It is essential for scaling up lab reactions to industrial production, where knowing precise quantities avoids wasted material and predicts expected yield.
Convert the given amount of each reactant into moles, then divide each by its coefficient in the balanced equation; whichever reactant gives the smallest resulting value is the limiting reactant that runs out first. The limiting reactant determines the maximum theoretical amount of product the reaction can produce.
Atom economy measures what percentage of the total mass of reactants ends up incorporated into the desired product, rather than being lost as byproducts. The Atom Economy Calculator uses the same mole-ratio stoichiometric relationships to determine how efficiently a reaction converts starting materials into useful product.
Determining an empirical formula from experimental composition data is itself a stoichiometry problem, since it requires converting mass measurements into mole ratios of each element present. The Empirical Formula Calculator applies the same mole-based logic used throughout stoichiometric calculations.
Theoretical yield is the maximum amount of product a reaction could produce as calculated from stoichiometry, assuming the limiting reactant is fully converted with no losses. Actual yield is the amount actually obtained in a real experiment, which is almost always lower due to side reactions, incomplete reactions, or product loss during purification.