__ A mole __is defined as quantity of a given substance that contains as many molecules or formula units as the number of atoms in exactly 12g of carbon-12.

The number of atoms in 12-g sample of carbon-12 is called **Avogadro's number **which is **6.02 x 10 ^{23}**.

Terms mole like a dozen refers to a perticular number of things.A dozen egg equals 12 eggs and a mole of ethanol equals 6.02 x 10^{23} molecules.

1 mole of H atoms = 6.02 x 10^{23 }H atoms

1 mole O atoms = 6.02 x 10^{23} O atoms

1 mole H2 molecules = 6.02 x 10^{23} H2 molecules

1 mole H2O molecule = 6.02 x 10^{23} H2O molecules

In using terms moles for ionic substances we mean the number of formula units of substance.For example,

A mole of sodium carbonate,Na2CO3 is a quantity containing 6.02 x 10^{23} Na2CO3 units.But each formula units of Na2CO3 contains two Na^{+ }ion and one CO3^{2- }ion.So, a mole of Na2CO3 also contains

2 x 6.02 x 10^{23} Na^{+ }ion and 1 x 6.02 x 10^{23} CO3^{2- }ion

**Mole and Molar mass**

**A **mole represents not only a specific number of particles but also a definite mass of a substance as represented by its formula.