1 mole of aluminium oxide, Al₂O₃ = 2  x 27 + 3 x 16 = 102 u = 102 g
102 g Al₂O₃ has = 6.023 x 10²³ Al₂O₃ molecules
0.051  g Al₂O₃ has = \(\frac { 6.023\times { 10 }^{ 23 } }{ 102 } \)
= 3.01 x 10²⁰ Al₂O₃ molecuels
1 molecule of Al₂O₃ gives = 2Al⁺⁺⁺ ions
Hence 0.051 g Al₂O₃ gives = 2 x 3.01 x 10²⁰ Al⁺⁺⁺ ions
=6.023 x 10²³ aluminium ions.

Molecular masses of H₂ = 1 +1 = 2 u
Molecular masses of O₂ = 16 +16 = 32 u
Molecular masses of Cl₂ = 35.5 + 35.5 = 71 u
Molecular masses of CO₂ = 12 + 32 = 44 u
Molecular masses of CH₄ = 12 x 2 + 1  x 6 = 30 u
Molecular masses of C₂H₆ = 12 x 2 + 1  x 4 = 28 u
Molecular masses of NH₃ = 1 x 14 + 1 x 3 = 17 u
Molecular masses of CH₃OH = 1 x 12 + 1 x 3 + 1 x 16 + 1 x 1 = 32 u

1 u = 1/12th of 1 atom of C-12
1 mole C-12 = 12 gram
12 gram C-12 = 6.022 x 10²³ atoms
or  6.022 x 10²³ atoms of C-12 = 12 grams
One atom of C-12 = \(\frac { 12 }{ 6.022\times { 10 }^{ 23 } } \)g
1 u = \(\frac { 12 }{ 6.022\times { 10 }^{ 23 } } \times \frac { 1 }{ 12 } \)
= 1.660302 x 10^-24 gram.

According to dalton's atomic theory, each element consists of atoms which are similar have same weights.Further atoms of one element combine with atoms of another element to form compounds.Let us suppose that x atoms of element A combine with y atoms of element B and the compound formed is AxBy.If  'a' stands for the atomic mass of A and 'b' for that of b. then
Percentage of A in the compound=\(\frac { ax\times 100 }{ ax+by } \)
Percentage of B in the compound=\(\frac { by\times 100 }{ ax+by } \)
Now a and b are fixed x and y are also fixed whole numbers according to atomic theory.Therefore. percentage of A and B in the compound is also constant.This shows that the composition of various elements in a compound is also fixed.This is the law of constant proportion.

The chemical formula of a compound is the symbolic representation of its composition.It gives the number and kinds of atoms which are chemically united in a given compound.For example, the chemical formula of hydrogen sulphide is HS.It tells that in hydrogen sulphide two hydrogen atoms and one sulphur atom are chemically united.

Sodium fluoride - Sodium cation and fluoride anion
potassium bromide - Potassium cation and bromide anion
calcium oxide - Calcium cation and oxide anion
silver sulphide - Silver cation and sulphide anion
magnesium oxide - Magnesium cation and oxygen anion.

Compounds containing charged species are known as ionic compounds.Such compounds are formed from combination of metals with non-meatls.For example, sodium chloride is an ionic compound.Its constituent particles charged sodium ions and negatively charged chloride ions.

(i) 1 Mole of Carbon atom = 6.022 x 10²³ atoms
6.022 x 10²³ atoms of carbon weigh = 12 g
1 atom of carbon weighs = \(\frac { 12 }{ 6.022\times { 10 }^{ 23 } } \\ \)  
= 1.99 x 10^-23
(ii) (a) n = 0.5 mol; M = 14 x 2 = 28 g; m = ?
m = n x M = 0.5 x 28 = 14.0 g
(b) n = 0.2 mol; M = 16 g; m = ?
m = n x M = 0.2 x 16 = 3.2 g
(c) n = 4 mol; M = 27 g; m = ?
m = n x M = 4 x 27 = 108 g

(i) Mass of 1 mole CO₂ = 12 + 32 = 44 u
Mass of 5 moles CO₂ = 44 x 5 = 220 u
Mass of 1 mole of H₂O = 2 + 16 = 18 u
Mass of 5 moles of H₂O = 5 x 18 = 90 u
(ii) 40 g calcium make = 1 mole
240 g calcium make = \(\frac { 240 }{ 40 } \) = 6 moles
24 g magnesium make = 1 mole
240 g magnesium make = \(\frac { 240 }{ 24 } \) = 10 moles
Ratio of 240 g calcium and 240 g magnesium in moles = \(\frac { 6 }{ 10 } \) or 3 : 5