Avogadro's Hypothesis

In trying to correlate Dalton's atomic theory with Gay Lussac's law of gaseous volume, Berzelius put forward the hypothesis that equal volumes of all gases contain equal number of atoms under similar conditions of temperature. However, it was found that even a fraction of an atom could be involved in certain chemical reactions. This went against Dalton's atomic theory. Therefore, Avogadro postulated the existence of molecules along with atoms as two kinds of ultimate particles. According to Avogadro's postulates,

• The smallest particle of an element, which may or may not have independent existence and takes part in a chemical reaction is an atom.

• The smallest particle of a substance (element or compound) capable of independent existence is called molecule.

Statement

Avogadro's law states that 'under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules'.

For example, if equal volumes of different gases like hydrogen, oxygen and chlorine are held in separate containers under similar conditions of temperature and pressure, then each container will contain the same number of molecules. The total mass and size of each gas in the container may differ from each other.

Fig:1.5 - Different gases under same conditions of pressure and temperature

Experimental evidences show that one mole of all gases under similar conditions of temperature and pressure occupy the same volume called molar volume (V_m). Molar volume of all gases under normal pressures (1 atm) and normal temperature (273K) have a value of 22.4 litre and contains 6.023 x 10²³ molecules. The number is called the Avogadro's number or Avogadro's constant and is denoted by N_A.

Avogadro's law is very useful in chemistry and has some important applications as in the deduction of the atomicity of the elementary gases, the derivation of relationship between molar mass and vapour density and determination of the molecular formula of a gaseous compound.