Amadeo Avogadro, conte di Quaregna e Ceretto was born August 9, 1776, in Turin, Italy and died July 9,1856, also in Turin. He was a physicist who first set forth the hypothesis known as Avogadro's Law, which states that equal volumes of gasses or 'vapours', at the same temperature and pressure contain the same number of molecules. This law explained why gasses chemically combine in simple proportions by volume and led Avogadro to believe that the elements of hydrogen, nitrogen, and oxygen exist as diatomic molecules in nature. A professor of higher physics at the University of Turin for many years, he stated by his hypothesis in 1811, but it was not generally accepted until after 1858, when the Italian chemist Stanislao Cannizzaro constructed a logical system of concepts of modern chemistry. Avogadro's number is the number of molecules in one gram-molecular-weight of any substance.

AVOGADRO'S LAW, a statement that under the same conditions of temperature and pressure, equal volumes of different gasses contain an equal number of molecules. This empirical relation, proposed by the Italian physicist Amadeo Avogadro in 1811, can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas. The law is approximately valid for real gases at sufficiently low pressures and high temperatures.
The specific number of molecules is given by Avogadro's constant. In the metre-kilogram-second (mks) system of units, Avogadro's constant is the number of molecules in one kilogram-mole of a substance, defined as the molecular weight in kilograms. The value of Avogadro's constant is 6.023 X 10²⁶ molecules per kilogram-mole in mks units or 6.023 X 10²³ molecules per gram-mole in the centimetre-gram-second (cgs) system of units. For example, the molecular weight of oxygen is 32, so that one kilogram-mole of oxygen has a mass of 32 kilograms and contains 6.023 X 10²⁶ molecules.
The volume occupied by one mole of gas is about 22.4 liters at standard temperature and pressure (STP), which are 0⁰ C and 1 atmosphere pressure, and is the same for all gases according to Avogadro's law.