Amount of substance



The amount of substance, n, of a sample or system is a physical quantity which is proportional to the number of elementary entities present. "Elementary entities" may be atoms, molecules, ions, electrons, or particles, the choice of which is dependent upon context and must be stated. Amount of substance is sometimes referred to as chemical amount or, incorrectly, as number of moles.[1]

The Avogadro constant, NA, which has a value[2] of 6.02214179(30)×1023 mol−1.

Rationale

Why use amount of substance instead of mass or volume to tell how much of a substance there is? This is because in chemical reactions, the reagents react oxygen.

See: Stoichiometry

Terminology

When quoting an amount of substance, it is necessary to specify the entity involved (unless there is no risk of ambiguity). One mole of empirical formula. For example:

amount of chloroform, CHCl3
amount of sodium, Na
amount of hydrogen (atoms), H
n(C2H4)

This can be considered to be a technical definition of the word "amount", a usage which is also found in the names of certain derived quantities (see below).

Derived quantities

When amount of substance enters into a derived quantity, it is usually as the denominator: such quantities are known as "molar quantities".[3] For example, the quantity which describes the volume occupied by a given amount of substance is called the chemistry.

The main derived quantity in which amount of substance enters into the numerator is amount of substance concentration, c. This name is often abbreviated to "amount concentration",[4] except in clinical chemistry where "substance concentration" is the preferred term[5] (to avoid any possible ambiguity with mass concentration). The name "molar concentration" is incorrect,[6] if commonly used.

See also

  • Amount fraction, x

References

  • Bureau International des Poids et Mesures (2006). The International System of Units (SI) (8th Edn). ISBN 92-822-2213-6. pp. 114–15.
  • International Union of Pure and Applied Chemistry. "amount of substance, n". Compendium of Chemical Terminology Internet edition.
  • International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 46. Electronic version.
  1. ^ To use "number of moles" to refer to amount of substance is no more correct than to use "number of seconds" to refer to time. The amount of substance is the same whatever unit is used to measure it.
  2. ^ International Council of Science Committee on Data for Science and Technology (2007). 2006 CODATA recommended values.
  3. ^ a b International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 7. Electronic version.
  4. ^ International Union of Pure and Applied Chemistry. "amount-of-substance concentration". Compendium of Chemical Terminology Internet edition.
  5. ^ International Union of Pure and Applied Chemistry (1996). "Glossary of Terms in Quantities and Units in Clinical Chemistry." Pure Appl. Chem. 68:957–1000.
  6. ^ "Molar concentration" should refer to a concentration per mole, i.e. an amount fraction. The use of "[[molar (unit)|]]" as a unit, equal to 1 mol/dm3, symbol M, is frequent, but not (as of May 2007) completely condoned by IUPAC: See International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 42 (n. 15). Electronic version.
 
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