⁕ What is mole?

↪ It is not possible to count the number of atoms or molecules in a definite amount of the substance.
↪ It is found that the number of atoms present in 1 gram atom of any element is equal to the number of molecules present in 1 g molecule of any substance.
↪ This number has been determined and found to be equal to 6.023 x 10²³  . It is called Avogadro's Number.

↪ Avogadro's number may be defined as the number of atoms present in one gram atom of the element or the number of molecules present in one gram molecules of the substance.

↪ The amount of substance containing Avogadro's number of atoms or molecules is called a mole (or simply written as mol).

↪ Thus, mole is defined as the amount of substance which contains the same number of discrete particles as the number of atoms present in 12 g of Carbon (C¹²).

⁕ Significance of a Mole

a. A mole gives the number of particles

↪ A mole contains Avogadro's number of discrete particles and it is equal to 6.023 x 10²³.

For examples:
1 mol of hydrogen atoms = 6.023 x 10²³ hydrogen atoms
1 mol of hydrogen molecules = 6.023 x 10²³ hydrogen molecules

1 mol of H₂ molecules contain 2 mol of H atoms.
i.e. 6.023 x 10²³ H₂ molecules contain 2 x 6.023 x 10²³ H atoms
                 1 mol of sodium ions = 6.023 x 10²³ sodium ions
                    1 mol of electrons   = 6.023 x 10²³ electrons

↪ Therefore, the number of moles and the number of discrete particles are related as

b. A mole gives the mass of a substance in gram.

↪ A mole is that amount of substance which has mass equal to gram atomic mass if the substance is molecular.

In other words,
↪ One mole of atoms = 6.023 x 10²³ atoms = Gram atomic mass of the element (molar mass of 1 mol of atoms)

For example:           1 mol of carbon atom = 6.023 x 10²³ atoms of carbon = 12 g
                                  1 mol of sodium atom = 6.023 x 10²³  atoms of sodium = 23 g
                                  1 mol of oxygen atom = 6.023 x 10²³ atoms of oxygen = 16 g

Similarly, 
                                  1 mol of molecules = 6.023 x 10²³ molecules
                                                                     = Gram molecular mass (molar mass of 1 mol of molecules)

For example: 
                               1 mol of oxygen molecules = 6.023 x 10²³ molecules of oxygen = 32 g
                                     1 mol of water molecules = 6.023 x 10²³ molecules of water = 18 g
                                            1 mol of CO₂ molecules = 6.023 x 10²³  molecules of CO₂ = 44g

↪ Here, the number of mole and mass in grams are related as

c. A mole gives the volume of vapor or gas

↪ In case of gases or vapors, a mole is the amount of the gas that a occupies a volume of 22.4 L at NTP.
↪ It means the volume occupied by 1 gram mole of an ideal gas or vapor at NTP is known as gram molecular volume (GMV) or simply molar volume which is 22.4 L.

For example:   1 mol of hydrogen gas = 22.4 L of hydrogen gas at NTP
                                 1 mol of oxygen gas = 22.4 L of oxygen gas at NTP
                                         1 mol of CO2 gas = 22.4 L of CO2 at NTP

↪ Here, the number of mole and volume of gas are related as