AVOGADRO'S HYPOTHESIS
Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
Importance of Avogadro's Law: The importance of Avogadro's Law lies in the fact that it enables us to change over directly from a statement about volume of gases to a statement about molecules of gases and vice-versa, e.g.,
(i) 1 volume ofhydrogen combines with 1 volume of chlorine to form 2 volumes of hydrogen chloride (temperature and pressure remain constant).
Applying the Law,
1 molecule ofhydrogen combines with 1 molecule of chlorine to give 2 molecules of hydrogen chloride.
Application of Avogadro's Hypothesis
Deducing the atomicity of elementary gases
Definition: The number of atoms contained in one molecule of the element is called its atomicity.
Atomicity of Hydrogen
In the reaction between hydrogen and chlorine, 1 vol. ofhydrogen combines with 1 vol. of chlorine to form 2 vols, ofhydrogen chloride.
H2 + Cl2 -> 2HC1
Hydrogen + Chlorine -> Hydrogen chloride
(lvol.) (1vol.) (2 vols.)
Applying Avogadro's Law
1 molecule + 1 molecule -> 2 molecules
From the above equation, we can say, each molecule of hydrogen chloride must contain atleast 1 atom of hydrogen.
Two molecules of hydrogen chloride, therefore, contain 2 atoms of hydrogen. These 2 atoms of hydrogen must have come from 1 molecule of hydrogen. Therefore, the molecule of hydrogen is diatomic and is written as Hr Thus the atomicity of hydrogen is 2.
Similarly it can be shown that a molecule of chlorine contains 2 atoms, i. e., the atomicity of chlorine is 2. From experimental evidence and similar arguments, it can be shown that oxygen and nitrogen also have 2 atoms per molecule, i.e., their molecules are diatomic.
The molecules of hydrogen, chlorine, oxygen, and nitrogen are diatomic, i.e., their atomicity is 2.