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The Mole and Avogadro's number
CHEMISTRY - Secondary 1
In This part we will study
The mole and Avogadro's number
The Italian scientist Amedeo Avogadro discovered that the number of particles (molecules, atoms , ions or formula units) that are found in one mole of any substance is a constant number, this number, later, was named after Avogadro's name (Avogadro's number) to honor him.
Avogadro's number: It is the number of atoms, molecules, formula units or ions which are found in one mole of substance and it equals 6.02 x 1023
(atom, molecule or ion) .
Mole: It is amount of substance that contains Avogadro's number of particles (molecules, atoms, ions or formula units).
Number of particles (molecules, atoms, ions or formula units) which are found in a definite amount of substance (number of moles) can be calculated in terms of Avogadro's number, using the relation :
Example (1) :
Calculate the number of sodium ions in 0.212 mol of sodium sulphate.
Solution : Na2SO4 "→ " 〖2Na〗^(+ ) " + " 〖"SO" 〗_"4" ^"2-"


Number of moles of Na+= 2 × 0.212 = 0.424 mol
Number of sodium ions Na+ = Number of ions moles x Avogadro's number
= 0.424 × 6.02 × 1023 = 2.55 × 1023 ions

Example (2) :
Calculate the number of sulphur dioxide molecules which are present in 32 g sample of it.
(S = 32 , O = 16)
Example (3):
Calculate the number of carbon atoms in 50 g of calcium carbonate. [Ca=40,C=12,O=16]

Example (4) :
A sample of a compound contains 5 g of hydrogen, 35 g of nitrogen and 60 g of oxygen, Calculate the number of hydrogen molecules in another sample of this compound, its mass equals 154.4 g


Best wishes to you
Mr.Ahmed Elbasha

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