IGCSEChemistryOxford AQARevision NotesQuantitative ChemistryUse of Amount of Substance In Relation to Masses of Pure SubstancesPercentage Composition

Percentage Composition (Oxford AQA IGCSE Chemistry)

Revision Note

Alexandra Brennan

Expertise

Chemistry

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 = open parentheses fraction numerator total space mass space of space element over denominator total space space mass space of space compound end fraction close parentheses × 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 = begin mathsize 14px style open parentheses fraction numerator 2 cross times 1 over denominator 18 end fraction close parentheses end style × 100 = 11.1%

  • Similarly, the percentage composition of oxygen is:

    • Percentage of oxygen = begin mathsize 14px style open parentheses fraction numerator 1 cross times 16 over denominator 18 end fraction close parentheses end style × 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 = begin mathsize 14px style stretchy left parenthesis fraction numerator total space mass space of space element over denominator total space space mass space of space compound end fraction stretchy right parenthesis end style × 100

  • So, the percentage composition of carbon in glucose is:

    • Percentage of carbon = open parentheses fraction numerator 6 cross times 12 over denominator 180 end fraction close parentheses × 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 = stretchy left parenthesis fraction numerator total space mass space of space element over denominator total space space mass space of space compound end fraction stretchy right parenthesis × 100

  • So, the percentage composition of nitrogen in ammonium nitrate is:

    • Careful: There are two nitrogen atoms in ammonium nitrate

    • Percentage of nitrogen = open parentheses fraction numerator 2 cross times 14 over denominator 80 end fraction close parentheses × 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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    Alexandra Brennan

    Author: Alexandra Brennan

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.