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SI units > Mole (mol)

Mole (mol)

MoleThe mole is the SI base unit for the amount of a substance. It is currently defined by comparison with the number of atoms in 12 g of Carbon-12.

Moles are measures of the amounts of substances. Accurate molar values are vital for effectively administering medicines and for all chemistry research. It is important in calculating the yield of industrial chemical reactions to know the amount of substances based on the number of atoms or molecules.

Amadeo Avogadro first proposed that the volume of a gas at a given temperature and pressure is proportional to the number of atoms, regardless of the gas.

As a result of his contributions to molecular theory, the number of atoms in 12 g of Carbon-12, used to define the mole, was named in his honour.


In 2019, the mole is expected to be redefined using Avogrado's number – 6.02×1023.

Did you know?

  • One mole of hydrogen atoms will be 6.02×1023 hydrogen atoms, in the same way that one mole of grapefruits would be 6.02×1023 grapefruits
  • The word 'mole' comes from the word 'molecule'
  • International Mole Day is celebrated on 23 October from 6:02 am to 6:02 pm

The science behind the unit

Currently, a mole is the amount of substance containing as many elementary entities as there are atoms in exactly 0.012 kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state. From May 2019, it is expected that one mole will contain exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number.

The mole is used to describe a practical quantity of material and is the link between the microscopic and macroscopic worlds, used to scale phenomena from the atomic up to 'relevant' sizes. As a result of the definition, the mole contains a defined number of entities, usually atoms or molecules. This number is the Avogadro constant (NA).

The current value for NA is:

6.022 141 79(30)x1023 mol-1

This number is a dauntingly large figure.

  • This number of sand grains would cover the United Kingdom to a depth of about 40 centimetres
  • There are about this number of human cells on Earth
  • It would take you twenty thousand million million years to count this number of coins (counting about one coin per second)

The mole is not realised as a unique physical artefact like the prototype kilogram. However, it can be realised by certain reproducible experimental methods known as 'primary methods'. One example is by weighing a sample of material of known composition.

Kilogram Metre Second Ampere Kelvin Mole Candela



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