Definition of the mole (IUPAC Recommendation 2017)

Roberto Marquardt, Juris Meija, Zoltán Mester, Marcy Towns, Ron Weir, Richard Davis and Jürgen Stohner

Pure and Applied Chemistry, 2018Volume 90, Issue 1, pp. 175–180

In 2011, the General Conference on Weights and Measures (CGPM) noted the intention of the International Committee for Weights and Measures (CIPM) to revise the entire International System of Units (SI) by linking all seven base units to seven fundamental physical constants. Of particular interest to chemists, new definitions for the kilogram and the mole have been proposed. A recent IUPAC Technical Report (Pure Appl. Chem. 89, 951 (2017); https://doi.org/10.1515/pac-2016-0808) discussed these new definitions in relation to immediate consequences for the chemical community. This IUPAC Recommendation on the preferred definition of the mole follows from that Technical Report. It supports a definition of the mole based on a specified number of elementary entities, in contrast to the present 1971 definition.

The new definition is:

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022  140 76 × 10²³ elementary entities. This number is the fixed numerical value of the Avogadro constant, N_A, when expressed in mol⁻¹, and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, or any other particle or specified group of particles.

This new definition is in contrast to the current definition, adopted in 1971, which relies on the mass of the kilogram. The new definition comes in advance of the anticipated revision of SI, announced in 2011 by the General Conference on Weights and Measures of the Bureau International des Poids et Mesures (BIPM), the international body responsible for the global comparability of measurements. The new SI will link all seven base units to fundamental physical constants. In November 2018, revised definitions of the kilogram, ampere, kelvin, and mole are expected to be approved by the CGPM. The revised definitions are expected to come into force on World Metrology Day, 20 May 2019.

https://doi.org/10.1515/pac-2017-0106