Avogadro’s hypothesis, first stated in 1811, was that equal volumes of gases under fixed conditions of temperature and pressure contain equal numbers of molecules. Half a century passed before the significance of this statement was fully appreciated, and Avogadro did not live to see his hypothesis confirmed. Today his name is given to the number of molecules in a gram-mole (or atoms in a gram-atom), or more precisely by SI standards the number of entities in one mole of carbon 12. This is known as Avogadro number or Avogadro constant, NA.
In the past NA has been measured experimentally by various methods, each producing different values of about 6 × 1023 per mol [Moelwyn-Hughes (1957)]. Today the accepted value is
Note that N A can also be derived from the relationship
Moelwyn-Hughes, E. A. (1957) Physical Chemistry, Pergamon Press.