Mole hero

The Unit – mole, mol (tīwhanga)

The mole is an SI unit that links the microscopic and macroscopic world. It allows scientists to measure large quantities of very small entities, such as atoms or molecules.

Initially, units called ‘gram-atom’ and ‘gram-molecule’ were used to specify amounts of chemical elements or compounds. These units were related to ‘atomic (or molecular) weights’, which in turn were linked to the atomic weight of oxygen. But inconsistencies began to emerge between physicists and chemists who each looked to different isotopes of oxygen. In 1959, an agreement established a unified scale of relative atomic masses, linked to the atomic weight of carbon 12.

As a result, a definition of the mole was adopted in 1971, and it was the amount of substance that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. This provided a more reliable way to specify quantities of chemical elements or compounds, now referred to as ‘the amount of substance’, and it greatly simplified quantitative studies in chemistry. This definition was used to determine the value of a fundamental constant of nature, the Avogadro constant (NA), which relates the number of entities to the amount of substance for any sample.

In May 2019, as part of the SI revision, the mole was redefined in terms of an exact value of NA. The new definition is as follows:

“The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol and is called the Avogadro number.”

The practical realisation of the mole has been laid out by Bureau International des Poids et Mesures (BIPM), and can be accessed here(external link).

Technical Capability

The mole is the only SI unit not realised at MSL. We refer customers to other laboratories for chemical measurement. The National Measurement Institute of Australia (NMIA) Chemical Proficiency Testing Laboratory(external link) can independently assess laboratory test methods and quality assurance programmes and provide independent evidence of the validity of chemical measurements.

Reference materials are available from NMIA(external link) (Australia), LGC (external link)(UK), IRMM(external link) (Belgium) or NIST(external link) (US).

Watch a short video about the mole here(external link).

Go to the Kilogram page.