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The 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.

The mole is a measure of the amount of substance. 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.

Amedeo 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 amount of substance of a system which contains as many entities as there are atoms in 0.012 kilgram of carbon 12, used to define the mole, was named in his honour.

In 2019, the mole will be redefined using Avogrado's number.

More about the redefinition in May 2019- If an atom was the size of a grapefruit, then an Avogadro number of them would have approximately the same volume as the Earth.
- The mole is used to quantify the number of atoms or molecules that can chemically react on a one-to-one (or simple whole number) basis, as this is much more useful than their mass or volume.

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 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, *N*_{A}, 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 (*N*_{A}).

The current value for *N*_{A} is:

6.022 141 79(30)x10^{23} mol^{-1}

This number is a dauntingly large figure.

- This number of sand grains would cover the United Kingdom to a depth of about a metre.
- 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.

© National Physical Laboratory 2018

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