In 1971, the General conference on weights and measures, adopted mole as the seventh basic
SI unit of the amount of a substance.
The term ‘mole’ is a Latin word which means heap or pile. A mole of atom is a collection of
atoms whose total weight is equal to the gram atomic weight. As equal number of moles of
different elements contain equal number of atoms, it is very easy to express the amount of a substance in terms of moles. Just as a dozen means twelve objects, in the similar fashion, a
mole is a collection of a ‘definite number’ of particles, viz., atoms, molecules, ions or electrons.
This ‘definite number’ is called Avogadro number which is equal to 6.023  1023. Thus, a
mole can be defined as, “the amount of a substance containing as many atoms, molecules, ions,
electrons or other elementary entities as there are carbon atoms in exactly 12g of C-12”.
Now-a-days, gram-molecules and gram-atom are termed as a mole of molecules and a
mole of atoms respectively, e.g., 1 gram-molecule of chlorine and 1 gram atom of
chlorine are expressed as 1 mole of Cl2
 and 1 mole of Cl respectively.
The number of moles can be calculated by a number of formulae depending upon the data
given :
No. of moles of molecules = Wt. of substance in g
Mol. wt. of substance
No. of moles of atoms = Wt. of element in g
At. wt. of the element
No. of moles of gas = Standard molar volume (i.e. 22.4L)
Volume of the gas at STP