GCSEScienceWJECScience (Double Award)ChemistryRevision NotesChemical Substances, Reactions & Essential ResourcesThe Nature of Substances & Chemical ReactionsPercentage Composition

Percentage Composition (WJEC GCSE Science (Double Award)): Revision Note

Exam code: 3430

Richard Boole

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WJEC Science (Double Award): ChemistryWJEC 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%

Examiner Tips and Tricks

  • 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 = open parentheses fraction numerator 2 cross times 1 over denominator 18 end fraction close parentheses x 100 = 11.1%

      • INCORRECT = begin mathsize 14px style stretchy left parenthesis fraction numerator 18 over denominator 2 cross times 1 end fraction stretchy right parenthesis end style x 100 = 900%

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%

Examiner Tips and Tricks

  • 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 

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    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry Content Creator

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.