The law of Avogadro ampere is also known as Avogadro’s simplest law, named after an Italian physicist and chemist, Amedeo Avogadro who first stated it in 1811. Three years later, however, Andre-Marie ampère formulates the same hypothesis, quite independently.

Avogadro’s Law asserts that there is always the same number of molecules in equal volumes of different perfect gases, provided that they are taken under the same conditions of temperature and pressure.

Towards the law of perfect gases

In other words, at pressure and at a given temperature, all the perfect gases have the same molar volume. Thus, under normal conditions of temperature and pressure – an atmosphere and 0 ° C – one mole of perfect gas invariably occupies 22.4 liters. Under the usual conditions of temperature and pressure – a bar and around 20 ° C – it occupies 24 liters.

The law of Avogadro-Ampere is part of a more general law known as the law of perfect gases. The latter argues that the quantity of gas material – that is to say the number of moles of this gas – is proportional to its volume and pressure and inversely proportional to its temperature. On the other hand, it does not depend on the nature of the gas. This law can be written:

n = PV / RT

where n is the number of mol expressed in mol, P is the pressure expressed in Pa, V is the volume expressed in m3 and T is the temperature expressed in K, R is a constant called constant of perfect gases; as a constant, it does not of course depend on the nature of the gas and is 8.31 J.K-1.mol-1