Avogadro’s Number:
The number of particles (atoms, ions, molecules, formula units) present in one mole of the respective substance is called Avogadro’s number or Avogadro’s constant.
It is denoted by NA. Its value is 6.022  1023
Example:
Na = 23g = 1mole = 6.022  1023 atoms.
H2O = 18g = 1mole = 6.022  1023 molecules.
NaCl = 58.5g = 1mole = 6.022  1023 formula units.
CO32- = 60g = 1mole = 6.022  1023 ions.
(i) The number of atoms in 1mole of an element is 6.02 x 1023 and is called Avogadro’s number 1.008g of Hydrogen = 1mole of hydrogen = atoms = 6.02 x 1023 atoms of H
23g of Na = 1mole of Na-atoms = 6.02 x 1023 atoms of Na
238g of U = 1mole of U-atoms = 6.02 x 1023 atoms of Cl
Number of atoms of an element =
(ii) No. of molecules in 1mole of a molecular compound is equal to 6.02 x 1023 and is called Avogadro’s number.
Examples:
18g H2O = 1mole H2O = 6.02 x 1023 H2O molecules.
180g C6H12O6 = 1 moles C6H12O6 = 6.02 x 1023 C6H12O6 molecules.
342g C12H22O11 = 1mole C12H22O11 = 6.02 x 1023 C12H22O11 molecules.
Hence, different masses of different compounds have equal number of molecules.
Number of molecules of compound = x NA
(iii) Number of formula units in 1mole of ionic compound is equal to 6.02 x 1023 and is called Avogadro’s number.
58.5g of NaCl = 1mole NaCl = 6.02 x 1023 formula units of NaCl.
74.5g of KCl = 1mole KCl = 6.02 x 1023 formula units of KCl.
111g CaCl2 = 1mole CaCl2 = 6.02 x 1023 formula units of CaCl2.
Number of formula units of ionic compounds = x NA.
(iv) Number of ions in 1mole of ionic specie is 6.02 x 1023 and is called Avogadro’s number.
17g = 1mole = 6.02 x 1023 ions
96g = 1mole = 6.02 x 1023 ions
96g = 1mole = 6.02 x 1023 ions
No. of ions = x NA.
Example:
(i) 18g of H2O is equal to 1mole and contain 6.02 x 1023 molecules of H2O, 2 x 6.02 x 1023 atoms of hydrogen and 1  6.02  1023 of oxygen.
(ii) 98g of H2SO4 is equal to 1mole and contains
• 2 x 6.02 x 1023 atoms of H–atoms.
• 6.02 x 1023 atoms of S–atoms.
• 4 x 6.02 x 1023 atoms of O–atoms.
The concept of Avogadro’s number can be applied to determine the number of positive ions, negative ions, total number of ion positive and negative charges from given amount of a substance which ionizes in water.
Example:
H2SO4 ionizes according to following equation.

If we have 9.8g (Molar mass = 98g/mole) of H2SO4 and it ionizes completely in water. According to above equation, it will ionize to give.
• 0.2 moles (0.2 x 6.02 x 1023 of H+ ions) of H+ ions
• 0.1 mole (0.1 x 6.02 x 1023 ions) of ions
• 0.2 moles of positive charges (0.2 x 6.02 x 1023 positive charges)
• 0.2 moles of negative charges (0.2 x 6.02 x 1023 negative charges)