Question

How can the atomicity of a compound be determined with the help of Avogadro’s law?

Answer 1

According to Avogadro’s law, equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.

The atomicity of a gas can be determined by comparing the volume of the gas that combines with or is produced from a known volume of another gas.

Example:

2 volumes of hydrogen combine with 1 volume of oxygen to form 2 volumes of water vapour:

\[\ce{2H2​ + O2​ -> 2H2​O}\]

Thus, one molecule of water contains 2 hydrogen atoms and 1 oxygen atom, so its atomicity is 3.