Avogadro's Law

AVOGADRO'S LAW (Avogadro), based on the hypothesis expressed in 1811 by the Italian physicist Avogadro, stating that "under identical conditions of temperature and pressure, equal volumes of all gases contain the same number of molecules". From this hypothesis, which later became a law after its confirmation from the standpoint of the kinetic theory of gases (see), it follows that the densities of gases measured under identical external conditions are related as their molecular weights (see). Consequently, the latter can be calculated from the densities of gases by taking the molecular weight of any gas as the unit of comparison. The "anomalous" densities found in some cases were easily explained by dissociation phenomena. From Avogadro's law it follows that the gram-molecule (see) of any gas occupies under identical external conditions the same volume (at 0° and 760 mm — 22.4 L). The number of molecules in a gram-molecule was later determined (6.064.10²⁵) and named Avogadro's number. Later, after J. H. van 't Hoff succeeded in proving the applicability of the laws of gases to solutions, Avogadro's law was used to determine the molecular weight of such substances which are difficult or impossible to obtain in the gaseous state (sugar). It was empirically proved that the osmotic pressure (see) of a dissolved substance is equal to the pressure of a gas which would be observed if, after the removal of the solvent, the dissolved substance filled the same volume in the form of a gas. From this it followed that the dissolved substance is in the solvent in the same molecular state as a gas. Consequently, Avogadro's law is applicable to solutions, namely: solutions showing the same osmotic pressure contain, at the same temperature, in an equal volume, the same amount of molecules of the dissolved substance; this amount is equal to the number of molecules contained in an equal volume of gas at the same temperature and pressure (van 't Hoff).