Percentages (O-level chemistry)

PERCENTAGE

We calculate the percentage of an element in a compound if we know the relative atomic masses e.g.

 

Example 1

 

What is the percentage of Fe in Iron II sulphate (FeSO₄. 7H₂O)

 

[Fe = 56, S = 32, 0 = 16, H = 1]

 

Solution:

R.F.M of FeSO₄.7H₂O

Fe   + S   + 4O + 7 (2H + O)  

 

56 + 32 + (4 x 16) + 7(2 + 16)

                        =          278

 

There are 56 parts of iron in 278 parts of the compound

Example 2

What is the percentage of nitrogen in ammonium sulphate? [(NH₄₎₂ SO₄

(N = 14, H = 1, S = 32 0 = 16)                                                         

 

Solution:

R.F.M of (NH4)₂SO₄   = 2(14 + 4)     + 32 + 16 X 4

                                       36 + 32   + 64

                                      = 132

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Mole concept (percentages)

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