In chemistry, the amount of substance (symbol n) in a given sample of matter is defined as a ratio (n = N/NA) between the number of elementary entities (N) and the Avogadro constant (NA).

The particular substance sampled may be specified using a subscript, e.g., the amount of sodium chloride (NaCl) would be denoted as nNaCl.

Historically, the mole was defined as the amount of substance in 12 grams of the carbon-12 isotope.

In chemistry, because of the law of multiple proportions, it is often much more convenient to work with amounts of substances (that is, number of moles or of molecules) than with masses (grams) or volumes (liters).

In terms of volume, the numbers would depend on the pressure and temperature of the reagents and products.

[3] To avoid ambiguity, the nature of the particles should be specified in any measurement of the amount of substance: thus, a sample of 1 mol of molecules of oxygen (O2) has a mass of about 32 grams, whereas a sample of 1 mol of atoms of oxygen (O) has a mass of about 16 grams.

Other methods of determining the amount of substance include the use of the molar volume or the measurement of electric charge.

[7] The molar mass of a substance depends not only on its molecular formula, but also on the distribution of isotopes of each chemical element present in it.

For example, the amount concentration of sodium chloride in ocean water is typically about 0.599 mol/L.

In chemistry, it is customary to read the unit "mol/L" as molar, and denote it by the symbol "M" (both following the numeric value).

Thus, for example, each liter of a "0.5 molar" or "0.5 M" solution of urea (CH4N2O) in water contains 0.5 moles of that molecule.

By extension, the amount concentration is also commonly called the molarity of the substance of interest in the solution.

In a mixture of gases, the partial pressure of each component is proportional to its molar fraction.

The alchemists, and especially the early metallurgists, probably had some notion of amount of substance, but there are no surviving records of any generalization of the idea beyond a set of recipes.

In 1758, Mikhail Lomonosov questioned the idea that mass was the only measure of the quantity of matter,[11] but he did so only in relation to his theories on gravitation.

The development of the concept of amount of substance was coincidental with, and vital to, the birth of modern chemistry.