Stoichiometry | Definition, Relationship, Examples

Definition of Stoichiometry

In chemistry, Stoichiometry determines the proportions in which elements or compounds react with one another.


Stoichiometry is the calculation of reactants and products in chemical reactions in chemistry.

The principles of Stoichiometry are based upon the law of conservation of mass. Matter can neither be created nor destroyed, so every element present in the product(s) of a chemical reaction must be equal to the mass of every element present in the reactant(s).

Discovery of Stoichiometry

The term stoichiometry was first used by Jeremias Benjamin Richter in 1792. When the first volume of Richter’s Stoichiometry or the Art of Measuring the Chemical Elements was published.

The term is derived from the Ancient Greek words στοιχεῖον stoicheion “element ” and μέτρον metron “measure”.

In patristic Greek, the word Stoichiometria was used by Nicephorus to refer to the number of line counts of the canonical New Testament and some of the Apocrypha.

Some other Definitions

Here are more definitions related to stoichiometry :


Because of the well-known relationship of moles to atomic weights, the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced equation. This is called composition stoichiometry.


Gas stoichiometry deals with reactions involving gases, where the gases are at a known temperature, pressure, and volume and can be assumed to be ideal gases.

For gases, the volume ratio is ideally the same by the ideal gas law, but the mass ratio of a single reaction has to be calculated from the molecular mass of the reactants and products. In practice, due to the existence of isotopes, molar masses are used instead when calculating the mass ratio.


A stoichiometric or stoichiometric ratio of a reagent is the optimum amount or ratio assuming that the reaction proceeds to completion:

  1. All of the reagents are consumed.
  2. There is no deficiency of the reagent.
  3. There is no excess of the reagent.


Chemical reactions, as macroscopic unit operations, consist of simply a very large number of elementary reactions, where a single molecule reacts with another molecule.

As the reacting molecules (or moieties) consist of a definite set of atoms in an integer ratio, the ratio between reactants in a complete reaction is also in the integer ratio.

A reaction may consume more than one molecule. And the stoichiometric number counts this number, defined as positive for products (added) and negative for reactants (removed).

The unsigned coefficients are generally referred to as the stoichiometric coefficients.


Different elements have a different atomic mass, and as collections of single atoms, molecules have a definite molar mass, measured with the unit mole (6.02 ×1023 individual molecules, Avogadro’s constant).

By definition, carbon-12 has a molar mass of 12 g/mol. Thus, to calculate the stoichiometry by mass, the number of molecules required for each reactant is expressed in moles. And multiplied by the molar mass to give the mass of each reactant per mole of reaction.

The mass ratios can be calculated by dividing each by the total in the whole reaction. Elements in their natural state are mixtures of isotopes of differing mass. Thus atomic masses and, therefore, molar masses are not exactly integers.

For instance, instead of an exact 14:3 proportion, 17.04 kg of ammonia consists of 14.01 kg of nitrogen and 3×1.01 kg of hydrogen. Because natural nitrogen includes a small amount of nitrogen-15, and natural hydrogen includes hydrogen-2 (deuterium).


A stoichiometric reactant is a reactant that is consumed in a reaction, as opposed to a catalytic reactant. Which is not consumed in the overall reaction because it reacts in one step and is regenerated in another step.

Stoichiometry is the field of chemistry concerned with the relative quantities of reactants and products in chemical reactions.

For any balanced chemical reaction, whole numbers (coefficients) are used to show the quantities (generally in moles ) of both the reactants and products.

For example, when oxygen and hydrogen react to produce water, one mole of oxygen reacts with two moles of hydrogen to produce two moles of water.

In addition, we can use stoichiometry to find quantities such as the amount of products that can produce with a given amount of reactants and per cent yield.

Upcoming concepts will explain how to calculate the amount of products that can produce given certain information.

The relationship between the products and reactants in a balanced chemical equation is very important in understanding the nature of the reaction. This relationship tells us what materials and how much of them are needed for a reaction to proceed.

Reaction stoichiometry describes the quantitative relationship among substances as they participate in various chemical reactions.

A stoichiometric chemical reaction is one where the quantities of the reactants and products are consumed, and none remain after the completion of the chemical reaction. Stoichiometry is useful for measuring chemical reactions such as those that occur in corrosion processes.

Relation of Stoichiometry with Law of Conservation of Mass

stoichiometry with law of conservation of mass

The rules followed in determining stoichiometric relationships are based on the law of conservation of mass and energy and the law of combining weights or volumes.

Stoichiometry is founded on the law of conservation of mass. Where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers.

This means that if the amounts of the separate reactants are known. Then we can calculate the amount of the product. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined. Then we can also calculate the amount of the other reactants.

This is illustrated in the image here, where the balanced equation is:

CH4 + 2O2 -> CO2 + 2H2O.

Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water. This particular chemical equation is an example of complete combustion.

Stoichiometry measures these quantitative relationships and determines the amount of products and reactants produced or needed in a given reaction.

Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry.

In the example above, reaction stoichiometry measures the relationship between the quantities of methane and oxygen that react to form carbon dioxide and water.

Stoichiometry helps us determine how much substance is needed or is present

Things that can be measured are:

  1. Reactants’ and Products mass
  2. Molecular weight
  3. Chemical equations
  4. Formulas

Stoichiometric Coefficient

stoichiometry coefficient

Stoichiometric coefficient or stoichiometric number is the number of molecules that participate in the reaction.

  • If you look at any balanced reaction, you will notice an equal number of elements on both sides of the equation. The stoichiometric coefficient is the number present in front of atoms, molecules or ions.
  • Stoichiometric coefficients can be fractions as well as whole numbers.
  • In essence, the coefficients help us to establish the mole ratio between reactants and products.
  • In a balanced reaction, both sides of the equation have the same number of elements. The stoichiometric coefficient is the number written in front of atoms, ions and molecules in a chemical reaction to balance the number of each element on both the reactant and product sides of the equation.
  • Though the stoichiometric coefficients can be fractions, whole numbers are frequently used and often preferred. These stoichiometric coefficients are useful since they establish the mole ratio between reactants and products. In the balanced equation: 2Na(s)+2HCl(aq)→2NaCl(aq)+H2(g)

Terminology in Stoichiometry

Terminology in Stoichiometry
  1. Stoichiometric Ratio: A positive integer ratio that relates the number of moles of reactants and products involved in a chemical reaction. It can determine this ratio from the coefficients of a balanced chemical equation.
  2. Stoichiometric Number: Equal to the stoichiometric coefficient in the balanced equation. But positive for products (because they are produced) and negative for reactants (since they are consumed).
  3. Reaction Stoichiometry: Describes the quantitative relationship between reactants and products within a given chemical reaction.
  4. Balanced Equation: When the quantity of each element is equal on both sides of the equation.

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