Percentage compositions

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

percentage by mass of the atoms of an element

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

percentage by molecular formula of the compound

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

2 comments:

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Mayank Shahabadee said...

The total mass of Na2SO3.10H2O is wrongly calculated. plz check