Atoms and molecules are so small in size that they cannot be counted individually. The chemists use the unit mole for counting atoms, molecules or ions. It is represented by n. A mole represents 6.022 × 10^{23} particles.

**Example. ** 1 mole of atoms = 6.022 × 10^{23} atoms.

1 mole of molecules = 6.022 × 10^{23} molecules The number of particles present in 1 mole of any substance is fixed i.e. 6.022 × 10^{23}.

This number is called Avogadro constant or Avogadro number. It is represented by No. 1 mole of atoms = 6.022 × 10^{23} atoms = Gram atomic mass or Molar mass of element

No. of moles = n

Given mass = m

Molar mass = M

Given number of particles = N

Avogadro number of particles = N_{0}

These relations can be interchanged as

Mass of element, m = n × M

or No. of particles of element, N = n × N_{0}

Similarly,

1 Mole of molecules = 6.022 × 10^{23} molecules

= Gram molecular mass of Molar mass

or m = n × M and N = n × N_{0}

1 mole of molecules = 6.022 × 10

This number is called Avogadro constant or Avogadro number. It is represented by No. 1 mole of atoms = 6.022 × 10

No. of moles = n

Given mass = m

Molar mass = M

Given number of particles = N

Avogadro number of particles = N

These relations can be interchanged as

Mass of element, m = n × M

or No. of particles of element, N = n × N

Similarly,

1 Mole of molecules = 6.022 × 10

= Gram molecular mass of Molar mass

or m = n × M and N = n × N

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