Amount of substance
extensive physical property
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.
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