In 1811 Amedeo Avogadro stated the hypothesis which we now call Avogadro's law:

equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. (See: this site for an English translation of his 1811 paper).

Thus the number of molecules in a specific volume of gas is independent of the size or mass of the gas molecules.

This important observation when combined with the observations of Avogadro's contemporaries:

the amount of moles is directly proportional to the volume of the gas;

the amount of moles is directly proportional to its pressure: from Boyle's law; and

the amount of moles is inversely proportional to its temperature: from Charles' law;

can be expressed in combined form:

Failed to parse (unknown function \propto): n \propto {PV \over T}

.

Including a proportionality constant, R, to make an equality instead of a proportion yields:

With pressure expressed in kPa, volume in Liters, and temperature in Kelvins; R (the gas constant) is equal to 8.314 (kPa × L)/(K × mol), and n represents the number of moles. This is also known as the ideal gas law: PV=nRT.

One mole of any gas occupies approximately 22.4 Liters (dm³) at STP. This is often referred to as the molar volume of a gas.

The number of molecules in one mole is called Avogadro's number: approximately 6.022 × 10²³ particles/mole.