In 1811 the Italian chemist Amedeo Avogadro formulated the law, which he found empirically, that under conditions of equal temperature and pressure the number of molecules of all gases in a given volume is the same.
The molecular weight of a gas molecule is the sum over the atomic weights of all its constituent atoms. Its mass is expressed through the unit gram-mole, which is the molecular weight in grams. The number of molecules in one gram-mole is the same for all gases and equal to 6.0221367.1023 (Avogadro's constant). One gram-mole of any gas occupies a volume of approximately 22.4 litres.
As an example, oxygen has the molecular weight 32. One gram-mole of oxygen thus weighs 32 grams, contains 6.0221367.1023 molecules and occupies a volume of 22.4 litres.
Avogadro's Law as not universally accepted until 1860, when the Italian chemist Stanislao Cannizzaro presentedt his system of atomic and molecular weight to the First International Chemical Congress in Karlsruhe, Germany, which he had developed from Avgadro's principles in 1858. Later experiments proved that Avogrado's Law is valid at sufficiently low pressures and high temperatures.
Avogadro's Law formed the basis for the introduction of atomic weight as a measure of the mass of atoms relative to a standard: One litre of oxygen weighs 16 times more than a litre of hydrogen, so the mass of an oxygen atom must be 16 times the mass of an atom of hydrogen. The carbon atom was chosen as the standard and given the atomic weight 12; the unit of atomic weight is thus 1/12 of the atomic weight of carbon.
When an absolute determination of the weight of the carbon atom became possible the relative scale was replaced with the absolute scale of the atomic mass unit (amu). One amu, also called one Dalton, equals 1.667.10-24 g.