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In routine chemical problems, it is necessary to consider quantities of substances in terms of the number of atoms or ions or molecules present. The unit devised by chemists to express number of atoms or ions or molecules is called the mole (mol).


A mole is defined as gram atomic mass or gram molecular mass or gram formula mass of any substance (atoms, molecules, ions) which contains 6.02 x 1023 particles.


The purpose of relating unit number of particles to the standard atomic masses is to provide a ready method of calculating the mass of a mole of any substance (molar mass).


Consider a mole of carbon-12 atoms and a mole of magnesium atoms. By definition, a mole of C-12 is 12 g of this substance. The atomic mass of magnesium is 24 which means each atom of magnesium is twice as heavy as carbon atom, it follows that mole of magnesium is 24g. Similarly a mole of oxygen atoms is 16 g and a mole of oxygen molecules is 32g.

Modem experimental methods for determining atoms, molecules and ions show that in one g mole of a substance, there are 6.02 x1023 particles. This huge number is called the Avogadro’s number. It is given in the honour of Amadeo Avogadro (1776-1856). It is denoted by NA.


In the light of Avogadro’s number, mole is comprehensively defined as the mass in grams of atoms or molecules or ions (gram atomic mass or gram molecular mass or gram formula mass) which contains Avogadro’s number of particles (6.02×1023).


A mole of hydrogen atom = g = 6.02 x l023 atoms

A mole of hydrogen molecule = 2 g = 6.02 x 1023 molecules

A mole of NaCl = 58.5g = l mole Na+ + 1 mol Cl-

= 6.02 x 1023 Na+ + 6.02 x 1023 Cl-

A mole of H2O = 18 g = 6.02 x 1023 molecules

2 moles of H2O = 36g = 2 x 6.02 x 1023 = 1.204 x 1024 molecules

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