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 (http://web.lemoyne.edu/~giunta/avogadro.html) 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:

[itex]n \propto {PV \over T}[itex].

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

[itex]n = {PV \over RT}[itex]

With pressure expressed in kilopascals, volume in liters, and temperature in kelvins; R (the gas constant) is equal to 8.314 (kPaL)/(Kmol), 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 per mole.de:Avogadrosches Gesetz fr:Loi d'Avogadro ja:アボガドロの法則 pl:Prawo Avogadra ru:Закон Авогадро uk:Авогадро закон zh:阿伏加德罗定律

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