Mole Concept and Avogadro's Number

In chemistry the term mole represents a pile or mass of atoms, molecules, ions or electrons. Just as common man measures quantity in terms of kilograms or dozens, a chemical scientist deals with a 'mole' of atoms, molecules, ions or electrons.

A mole is the amount of a substance that contains the same number of units of atoms or molecules or ions, as there are atoms in 12 g of the carbon

The 12 g of carbon contains a certain number of atoms of carbon. This number is estimated to be 6.023 x 10²³ and is called Avogadro's number. Thus 12 g of the carbon is one mole of carbon.

Since the number of atoms in 12 g of carbon is called Avogadro's number, a mole may be redefined as the amount of a substance, which contains Avogadro number of molecules.

Avogadro's Number

Avogadro's number is the number of elementary units (atoms, molecules, or ions) present in one mole of a substance, and it is equal to 6.023 x 10²³.

The concept of a mole being equal to a number 6.023 x 10²³ is akin to the concept of a dozen being equal to a number 12.

A mole of atoms consists of Avogadro's number of atoms, and has a mass equal to one gram atomic mass of that element. Therefore, the following illustrations hold true:

• One mole of oxygen atoms contains 6.023 x 10²³ atoms of oxygen and has a mass of 16 g.

• One mole of sodium atoms contains 6.023 x 10²³ atoms of sodium, and has a mass of 23 g.

Similarly one mole of molecule of a substance has Avogadro's number, 6.023 x 10²³, of molecules and has a mass

     equal to one gram molecular mass of that substance.

• One mole of oxygen molecules contains 6.023 x 10²³ molecules of oxygen and has a mass of 32 g.

• One mole of sodium (mono atomic) contains 6.032 x 10²³ molecules (or atoms, as it is mono atomic) and has a mass of 23 g.

• One mole of sulphuric acid contains 6.023 x 10²³ molecules of sulphuric acid and has a mass of 98 g.

• One mole of carbon dioxide contains 6.023 x 10²³ molecules of carbon dioxide, and has a mass of 44 g.

• One mole of an ionic substance like sodium chloride or calcium chloride contains 6.023 x 10²³ formula units.

• One mole of sodium chloride contains 6.023 x 10²³ formula units i.e., 6.023 x 10²³ sodium ions and 6.023 x 10²³ chloride ions. It has a mass of 58.5 g (i.e., 23 + 35.5) g.

• One mole of calcium chloride contains 6.023 x 10²³ formula units i.e., 6.023 x 10²³ calcium ions and [2 (6.023 x 10²³)] chloride ions. It has a mass of 111 g.

• One mole of aluminium chloride contains 6.023 x 10²³ formula units of it, and contains 6.023 x 10²³ aluminium ions and (3 x 6.023 x 10²³) chloride ions. It has a mass of 133.5 (27 + {3 x 35.5}).

Normally, mass of 1 mole of a substance is called its molar mass. It is expressed as g/mol. For example molar mass

    of water is equal to 18 g/mol.)

Remember :

Mass of a mole of substance is equal to the molecular mass of a substance expressed in grams,

Example: One mole of oxygen = 16 x 2 = 32 g

One mole of carbon dioxide = (12 x 1) + (2 x 16) = 44 g

In fact 'mole' is the unit of the physical quantity 'amount' of a substance. Mole is abbreviated as 'mol'. When 'mole' is written as a unit and written with numerals, it is written as only 'mol'. For e.g., 1 mol, 4.5 mol, etc.

---