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.

This provided a more reliable way to specify quantities of chemical elements or compounds, now referred to as ‘the amount of substance’. Because this is proportional to the number of, for example, atoms, in a sample, it provides a universal constant which is the same for all samples. The unit of amount of substance is called the mole, and its official definition, adopted in 1971, states:

“The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is ‘mol’.

When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.”

Therefore, the mole allows scientists to count the atoms in a substance, simply by weighing a sample of that substance. The definition also determines the value of the Avogadro constant (NA), which relates the number of entities to amount of substance for any sample. Because NA = 6.022 141 79(30) x 1023 mol-1, it follows that 1 mol of carbon atoms = 6.022 x 1023 carbon atoms. The mole greatly simplifies quantitative studies in chemistry.

The mole is currently being considered for redefinition due to its link with another SI unit, the kilogram . It is expected that this will result in a fixed value of the Avogadro constant.

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).

Go to the Kilogram page.