The N - value is very large. So, for convenience the number of molecules are expressed in terms of another factor called mole. The mole is one of the basic units and one mole is defined as the number of atoms in 12g of Carbon-12.

            By experiments, that number is found to be per mole and is called Avogadro's number in honour of Italian scientist Amadeo Avogadro (1776-1856). So equn(3) is expressed in terms of moles

                                      
                                             ...........................(4)
where .