How do you calculate the percent composition of an element in a compound

What is Percentage Composition?

Percentage composition is defined as ” the number of parts of an element in 100 parts of a compound “

Or in simple words:

The percentage amount of all elements in the compound is called percentage composition.

Percentage Composition Calculation Formulae:

%age of element Mass of element in the compound   X 100

                       Mass or molar mass of compound


%age of elementAtomic mass X Number of moles X 100

                        molar mass of compound


%age of elementAtomic mass X Number of atoms X 100

                  molar mass of compound Avogadro’s number

Related Important Question:

What is the percentage composition of water?

h2o diagram

The percentage composition of water can be calculated as follow:

Molar mass of water (H2O)= ( 2 X1) + (1 X 16)

= 2+ 16

= 18 g/mol

we know that :

%age of element Mass of element in the compound   X 100

                               Mass or molar mass of compound

                           %age of H =     2   X 100


= 11.11%

                           %age of O =     16   X 100


                                                = 88.89%


Aluminium sulphate hydrate [Al2(SO4)3.x H2O] contains 8.20% of Al by mass, calculate x.


Formula mass of Al2(SO4)3.x H2O

     = 27×2 + 32×2 + 16×12 + x(1×2 + 16)


Formula mass of Al2(SO4)3.x H2O     

= 54 + 96 + 192 + x(18)


 Formula mass of Al2(SO4)3.x H2O    = 342 + 18x

Percentage of Al in Al2(SO4)3.x H2O = 8.20%


  Percentage of Al in Al2(SO4)3.x H2O = 

    Amount of Al in Al2(SO4)3.x H2O x 100

                                                                                    Formula mass of Al2(SO4)3.x H2O

8.20     =          54            x 100

342 + 18x


8.20 (342 + 18x)    =   5400


2804.40 + 147.60x = 5400


147.60x = 5400- 2804.40


147.60x = 2595.60


  x = 2595.60 / 147.60 =   17.59   ≈ 18

Q. Calculate the percentage composition of a compound. ( MgSO4 )

Molar mass of MgSO4 = 1 x 24 + 1 x 32 + 4 x 16 = 24 + 32 + 64 = 120

% age of Mg =   Mass of Mg             x 100   


   =  24  x 100    = 20 %



% age of S =         Mass of S            x 100 


      =  32  x 100    = 26.67 %


 % age of O =         Mass of O           x 100   


64  x 100    = 53.33 % 


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