the principle that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Thus, the molar volume of all ideal gases at 0° C and a pressure of 1 atm. is 22.4 liters.
Origin of Avogadro's law
First recorded in 1870-75; named after A. Avogadro
(ä'və-gä'drōz) The principle that equal volumes of all gases under identical conditions of pressure and temperature contain the same number of molecules. Avogadro's law is true only for ideal gases (gases in which there is no interaction between the individual molecules).