 # 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

OR:

%age of element =     Atomic mass X Number of moles X 100

molar mass of compound

OR:

%age of element =     Atomic mass X Number of atoms X 100

molar mass of compound Avogadro’s number

Related Important Question:

## What is the percentage composition of water?

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

18

= 11.11%

%age of O =     16   X 100

18

= 88.89%

## Numericals:

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

Solution:

Formula mass of Al2(SO4)3.x H2O      = 27×2 + 32×2 + 16×12 + x(1×2 + 16)

or         Formula mass of Al2(SO4)3.x H2O      = 54 + 96 + 192 + x(18)

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

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

But      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

Or                                            8.20 (342 + 18x)    =   5400

Or                                            2804.40 + 147.60x = 5400

Or                                            147.60x = 5400- 2804.40

Or                                             147.60x = 2595.60

Or                                            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 %

Molar mass of MgSO4     120

% age of S =         Mass of S            x 100       =  32  x 100    = 26.67 %

Molar mass of MgSO4     120

% age of O =         Mass of O           x 100       =  64  x 100    = 53.33 %

Molar mass of MgSO4     120

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