Avogadro's law states that under the same conditions of temperature and pressure, equal volumes of different gases contain an equal number of molecules. This relation holds exactly for ideal gases. The law is approximately valid for real gases at sufficiently low pressures and high temperatures.
The specific number of molecules in one mole of a substance is indicated by NA, a quantity called Avogadro's number. Its value is ≈ 6.022 × 1023 For example, the molecular weight of oxygen is 32.00, so that one mole of oxygen has a mass of 32.00 grams and contains NA molecules.
The volume occupied by one mole of an ideal gas is about 22.4 liter (0.791 cubic feet) at 0° C and 1 atmosphere.
The law was first proposed in 1811 by the Italian physicist Amedeo Avogadro, but it was not generally accepted until after 1858, when the Italian chemist, Stanislao Cannizzaro constructed a logical system of chemistry based on it.