Avogadro's law is a principle stated by the Italian chemist Amedeo Avogadro (1776-1856) in 1811.

The avagadro's law states that under equal conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules.

Avogadro's number is 6.022 * 10 ²³. This means that 12 grams of carbon-12 contains 6.022 * 10 ²³ carbon-12 atoms.

For a given mass of an ideal gas, the volume and amount (moles) of the gas are directly proportional if the temperature and pressure are constant.

The avagadro's law is expressed as: $V\propto n\mathrm{or}\frac{V}{n}=k$
where:
V is the volume of the gas.
n is the amount of substance of the gas (measured in moles).
k is a constant.

Because the formula is equal to a constant, Avagadro's law can be rewritten as: $\frac{V1}{n_1}=\frac{V2}{n_2}$

Example 1:
How many moles are there in one atom?

$\mathrm{No.\; of\; moles}=\frac{1\mathrm{atom}}{6.022x{10}^{23}\mathrm{atoms/mole}}=1.66x{10}^{-24}\mathrm{moles}$