Percentage Composition (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
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 = × 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 = × 100 = 11.1%
Similarly, the percentage composition of oxygen is:
Percentage of oxygen = × 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%
Worked Example
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 = × 100
So, the percentage composition of carbon in glucose is:
Percentage of carbon = × 100 = 40%
Worked Example
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 = × 100
So, the percentage composition of nitrogen in ammonium nitrate is:
Careful: There are two nitrogen atoms in ammonium nitrate
Percentage of nitrogen = × 100 = 35%
Exam Tip
Make sure you calculate the percentage composition using the total mass of the element. For example, a common mistake with ammonium nitrate is doing the calculation for only one atom of nitrogen. This would lose a mark in an exam
Show ALL your working out. If you make a mistake in the calculation, you could still score a mark for your workings.
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