Percentage Composition

Section 1 of 6The idea

What it tells you

• What it tells you The percentage composition of a compound is the share of its total mass that comes from each element, written as a percentage.
• It lets you compare, for instance, how much nitrogen different fertilisers supply.

The equation

• The equation Percentage by mass of an element = (A_r × number of atoms of that element ÷ M_r of the compound) × 100
• The part always goes on top and the whole on the bottom, so the answer can never exceed 100%.

A built-in check

• A built-in check If you calculate the percentage for every element in the compound, the values should add up to 100%, allowing for rounding.

Section 2 of 6Method

1

Write the formula of the compound and find the relative atomic mass of each element from the Periodic Table.

2

Work out the M_r of the whole compound, counting every atom, including those inside brackets.

3

Work out the total mass of the chosen element by multiplying its A_r by the number of its atoms in the formula.

4

Divide the mass of the element by the M_r of the compound and multiply by 100.

5

Check the answer is below 100% and show every step of the working.

Section 3 of 6Worked examples

Compound	Mr working	Percentage of the element
Water, H2O	(2 × 1) + 16 = 18	Hydrogen: 2 ÷ 18 × 100 = 11.1%; oxygen: 16 ÷ 18 × 100 = 88.9%; total 100%
Glucose, C6H12O6	(6 × 12) + (12 × 1) + (6 × 16) = 180	Carbon: 72 ÷ 180 × 100 = 40%
Ammonium nitrate, NH4NO3	14 + 4 + 14 + 48 = 80	Nitrogen: 28 ÷ 80 × 100 = 35%
Magnesium oxide, MgO	24 + 16 = 40	Magnesium: 24 ÷ 40 × 100 = 60%

Section 4 of 6Spotting common errors

1Count every atom

• Count every atom In NH₄NO₃ the nitrogen appears in two places, giving two nitrogen atoms in total.

2Use the total mass

• Use the total mass Multiply the A_r by the number of atoms before dividing.
  • Using one atom only loses the mark.

3Part over whole

• Part over whole If the fraction is upside down you may get an impossible answer such as 900%.

4Check and show working

• Check and show working A result above 100% means an error. Written steps can still earn method marks.

Section 5 of 6Key terms

Term	Meaning
Percentage composition	The mass of an element in a compound expressed as a percentage of the total mass of the compound.
Relative atomic mass (Ar)	The mass of an atom of an element compared with one twelfth of a carbon-12 atom.
Relative formula mass (Mr)	The sum of the relative atomic masses of all atoms in the formula.
Fertiliser	A substance added to soil to supply elements such as nitrogen that plants need.

Section 6 of 6Exam technique

1

Using the A_r of one atom when the formula contains more than one, for example one nitrogen atom in ammonium nitrate instead of two.

2

Dividing the whole by the part so that the answer is over 100%.

3

Forgetting to multiply by 100 and leaving the answer as a decimal fraction.

4

Leaving out atoms inside brackets when working out the M_r of the compound.