Chemists need to know the number of atoms present in a given sample of matter. They can
measure mass, but they
cannot count atoms.
This is because atoms are VERY small, and there are a VERY large number of atoms in any sample of matter that is large enough to see.
To
overcome this
problem, chemists have defined the
mole (
n), a unit of chemical amount
. One mole of a substance is the mass of substance that contains same number of entities as there are atoms in 12 g of
12C.
This number has been experimentally shown to be 6.022 × 10
23.
The
Avogadro constant (
) is equal to
6.022 × 1023 mol–1 (read as 6.022 times 1023 per mole). The mole is a counting unit like dozen or pair. The constant for dozen would be 12 doz
–1; for pair it would be 2 pr
–1.
The entity for a dozen eggs would be an egg. The entity for a pair of shoes would be a shoe.
Because the Avogadro constant is so large, the mole is
most suitable for describing very
large numbers of
very small entities. In a helium-filled balloon there are roughly 3.8 × 1023 atoms. This is described more conveniently as 0.625 mole.
If a sample of a substance contains a known amount in moles of a specified entity, the number of entities present can be calculated.
amount in mol of entity × Avogadro constant = number of entities
In chemistry the entities are usually molecules, atoms or ions.
To calculate the number of helium atoms in 0.800 mol He:
0.800 mol × 6.022 × 1023 mol–1 = 4.82 × 1023
Note that the result is a pure number because mol cancels with mol–1 (mol × mol–1 = 1).