Percentage compositions

Percentage composition is the percentage by mass of the atoms of an element present in one mole of a compound.

This may also be obtained from the molecular formula of the compound by using the relationship

Example: 13

Calculate the percentage composition of each of the elements present in pure calcium carbonate (Ca = 40, C = 12, O = 16).

Solution:

Mass of one mole of pure calcium carbonate (CaCO₃)

= Mass of calcium + Mass of carbon + Mass of oxygen

= (40 x 1) + (12 x 1) + (16 x 3)

= 40 + 12 + 48

= 100 g

Example: 14

Calculate the percentage loss of mass when sodium carbonate crystals are strongly heated (Na=23, C=12, O=16, H=1).

Solution:

Mass of crystals of Na₂SO₃.10H₂O = 286

Mass of water of crystallisation = 180

Loss of mass = 180

= 62.94%

Example: 15

Calculate the percentage composition of the elements present in pure calcium sulphate. (Ca = 40, S= 32, O =16).

Solution:

Mass of 1 mole of CaSO₄ = 40 + 32 + (16 x 4)

=40 + 32 + 64 = 136

Example: 16

Calculate the percentage composition of the elements present in sodium sulphate decahydrate (Na=23, S=32, O=16, H=1)

Solution:

Mass of 1 mole of Na₂SO₄.10H₂O

= (23 x 2) + (32 x 1) + (16 x 4) + 10 (1 x 2 + 16)

or, combining the oxygen atoms

= 46 + 32 + 64 + 20 + 160 = 322