MOLE AND AVOGADRO'S NUMBER

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

A mole ts defined as gram atomic mass or gram molecular mass 

or gram formula mass of any substance (atoms, molecules, ions) which contains 6.02 x 10“ particles. 

The purpose of relating unit number of particles w 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 substarice. 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 molccules is 32g. Moder experimental methods for the determining atoms, molecules and ions 

show that in one £ mele of a substance, there are 6.02x10” 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 N,. 

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 (6.02x10”). 

Example: A mole of hydrogen atom 

1 g = 6.02 x 10 atoms 

2g=6.02x 10" molecules = 58.5g = 1 mole Na’ + 1 mole Cl 

=6.02 x 10® Na’ + 6.02 x 10° Cl A mole of H,O = 18 g = 6.02x10” molecules 

2 moles of H,O = 36g = 2 x 6.02 x 10” = 1.204 x 10* molecules 

A mole of hydrogen molecule A mole of NaCl