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). It is one of the seven base quantities of the International System of Units, SI. The unit of amount of substance in the SI is the mole (symbol: mol), a base unit. Since 2019, the mole has been defined such that the value of the Avogadro constant NA is exactly 6.02214076×1023 mol−1, defining a macroscopic unit convenient for use in laboratory-scale chemistry. The elementary entities are usually molecules, atoms, ions, or ion pairs of a specified kind. The particular substance sampled may be specified using a subscript or in parentheses, e.g., the amount of sodium chloride (NaCl) could be denoted as nNaCl or n(NaCl). Sometimes, the amount of substance is referred to as the chemical amount or, informally, as the "number of moles" in a given sample of matter. The latter term is deprecated by the IUPAC because, for a substance X, the correct meaning of "number of moles" is n(X)/mol. The amount of substance in a sample can be calculated from measured quantities, such as mass or volume, given the molar mass of the substance or the molar volume of an ideal gas at a given temperature and pressure.
Usage Because of the way the mole and the dalton are defined, the mass in grams of one mole of a chemical compound is numerically very nearly equal to the mass of one molecule or formula unit of the compound in daltons. For example, a single molecule of water has a mass of about 18.0153 daltons on average, whereas a mole of water (which contains 6.02214076×1023 water molecules) has a mass of about 18.0153 grams on average. The molar mass of an isotope in grams per mole is approximately equal to the mass number. Before the mole was redefined in 2019, this equality was exact by definition for carbon-12. In chemistry, because of the law of multiple proportions, it is often more convenient to work with amounts of substances denominated in moles, than with masses (grams) or volumes (liters). For example, the chemical fact "1 molecule of oxygen (O2) will react with 2 molecules of hydrogen (H2) to make 2 molecules of water (H2O)" can also be stated as "1 mole of O2 will react with 2 moles of H2 to form 2 moles of water". The same chemical fact, expressed in terms of masses, would be "32.0 g of oxygen (1 mole of O2) will react with approximately 4.0 g hydrogen (2 moles of H2) to make approximately 36.0 g of water (2 moles of H2O)" (and the numbers would depend on the isotopic composition of the reagents). In terms of volume, the numbers would depend on the pressure and temperature of the reagents and products, although the volume of an ideal gas is proportional to the amount in moles or number of molecules at constant temperature and pressure. For the same reasons, the concentrations of reagents and products in solution are often specified in moles per liter, rather than grams per liter. The amount of substance is also a convenient concept in thermodynamics. For example, the pressure of a certain quantity of a noble gas in a recipient of a given volume, at a given temperature, is directly related to the number of molecules in the gas (through the ideal gas law), not to its mass. This technical sense of the term "amount of substance" should not be confused with the general sense of "amount" in the English language. The latter may refer to other measurements such as mass or volume, rather than the number of particles. There are proposals to replace "amount of substance" with more easily distinguishable terms, such as enplethy and stoichiometric amount. The IUPAC recommends that "amount of substance" should be used instead of "number of moles", just as the quantity mass should not be called "number of (kilo)grams".
Nature of the particles
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 32.00 g, whereas a sample of 1 mol of atoms of oxygen (O) has a mass of about 16.00 g.
Derived quantities
Molar quantities (per mole)
The quotient of some extensive physical quantity of a homogeneous sample by its amount of substance is an intensive property of the substance, usually named by the prefix "molar" or the suffix "per mole". For example, the quotient of the mass of a sample by its amount of substance is its molar mass, for which the SI unit kilogram per mole or gram per mole may be used. This is about 18.015 g/mol for water, and 55.845 g/mol for iron. Similarly for volume, one gets the molar volume, which is about 18.069 millilitres per mole for liquid water and 7.092 mL/mol for iron at room temperature. From the heat capacity, one gets the molar heat capacity, which is about 75.385 J/(K⋅mol) for water and about 25.10 J/(K⋅mol) for iron.
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