Is a principle stated in 1811 by the Italian chemist Amedeo Avogadro (1776-1856) that “equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties”. This number (Avogadro’s number) is 6.023 X 1023. It is the number of molecules of any gas present in a volume of 22.41 L and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine