Chemistry often needs to count microscopic entities in macroscopic samples. The SI quantity amount of substance is measured in moles.

One mole contains exactly Avogadro's number

$$N_A=6.02214076\times10^{23}$$

specified entities. The entities must be stated: atoms, molecules, ions, electrons, formula units, or another defined object.

If a sample contains $N$ entities, its amount is

$$n=\frac{N}{N_A}.$$

A molar quantity is an extensive quantity divided by amount of substance. For mass,

$$M=\frac{m}{n}$$

is molar mass, commonly expressed in $\mathrm{g/mol}$ or $\mathrm{kg/mol}$.

Example

Water has molar mass about $18.0,\mathrm{g/mol}$. A $36.0,\mathrm g$ sample therefore contains

$$n=\frac{36.0}{18.0}=2.00,\mathrm{mol}$$

of $\mathrm{H_2O}$ molecules, corresponding to

$$N=2.00N_A\approx1.20\times10^{24}$$

molecules.

The mole is a counting scale, not a special kind of particle. It connects atomic-scale stoichiometry with measurable mass, charge, energy and concentration.