Amedeo Avogadro, an Italian scientist, analysed Gay-Lussac’s *law of combining volumes* and hypothesised in 1811 that

**Equal volumes of all gases at the same temperature and pressure have the same number of molecules**

This became known as ** Avogadro’s law** or

*Avogadro’s principle*. It implies that for a given mass of a gas at constant temperature and pressure, its volume is directly proportional to its amount,

*n*, i.e.

where *k _{4}* is the proportionality constant.

Since *V _{1}* =

*k*

_{4}*n*and

_{1}*V*=

_{2}*k*

_{4}*n*,

_{2}*Avogadro’s law*can also be expressed as