Percentage Composition

The percentage composition of any compound is a way to express the mass of each element as a percentage of the total mass of the compound
The equation for percentage composition is:

Percentage composition = $(\frac{total mass of element}{total mass of compound})$ × 100

For example, in water:
- Water is a simple molecule with the chemical formula H₂O
- So, water is made of two hydrogen (H) atoms and one oxygen (O) atom
- From the Periodic Table, the relative atomic mass of:
  - Hydrogen = 1
  - Oxygen = 16
- Therefore, the total mass of water is:
  - (2 × 1) + 16 = 18
To find the percentage composition of hydrogen:
- Percentage of hydrogen = $(\frac{2 \times 1}{18})$ × 100 = 11.1%
Similarly, the percentage composition of oxygen is:
- Percentage of oxygen = $(\frac{1 \times 16}{18})$ × 100 = 88.9%
Note: The total percentage by mass of all the elements should add up to 100%, e.g. 11.1% + 89.9% = 100%

The most common incorrect answer for percentage composition are values above 100%
This is not possible and usually happens when the numbers in the calculation are "upside down"
- For example, the percentage composition of hydrogen in water:
  - CORRECT = $(\frac{2 \times 1}{18})$ x 100 = 11.1%
  - INCORRECT = $(\frac{18}{2 \times 1})$ x 100 = 900%

Calculate the percentage of carbon in the formula of glucose, C₆H₁₂O₆.

Answer:

From the Periodic Table, the relative atomic masses are:
- Carbon = 12
- Hydrogen = 1
- Oxygen = 16
The total mass of glucose is:
- (6 × 12) + (12 × 1) + (6 × 16) = 180
The equation for percentage composition is:
- Percentage composition = $(\frac{total mass of element}{total mass of compound})$ × 100

So, the percentage composition of carbon in glucose is:
- Percentage of carbon = $(\frac{6 \times 12}{180})$ × 100 = 40%

The chemical formula of the fertiliser ammonium nitrate is NH₄NO₃. Calculate the percentage by mass of nitrogen in ammonium nitrate.

Answer:

From the Periodic Table, the relative atomic masses are:
- Nitrogen = 14
- Hydrogen = 1
- Oxygen = 16
The total mass of ammonium nitrate is:
- (1 x 14) + (4 × 1) + (1 x 14) + (3 × 16) = 80
The equation for percentage composition is:
- Percentage Composition = $(\frac{total mass of element}{total mass of compound})$ × 100

So, the percentage composition of nitrogen in ammonium nitrate is:
- Careful: There are two nitrogen atoms in ammonium nitrate
- Percentage of nitrogen = $(\frac{2 \times 14}{80})$ × 100 = 35%

Make sure that you account for all the atoms of the element in the question!
Examiners often pick chemicals with more than one atom of an element as both worked examples show
- The NH₄NO₃ worked example is more challenging
- The two nitrogen atoms are written in different parts of the compound
- This means that the calculations seen in exams often didn't include both of them
Another relatively common question was about SnF₂, which contains 2 fluorine atoms, in toothpaste

Percentage Composition (WJEC GCSE Chemistry: Combined Science)