Percentage Composition (WJEC GCSE Chemistry): Revision Note

• 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 = $\left(\frac{2\times 1}{18}\right)$ x 100 = 11.1%

    • INCORRECT = $\left(\frac{18}{2\times 1}\right)$ 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 = $\left(\frac{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{element}}{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{compound}}\right)$ × 100

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

  • Percentage of carbon = $\left(\frac{6\times 12}{180}\right)$ × 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 = $\left(\frac{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{element}}{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{compound}}\right)$ × 100

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

  • Careful: There are two nitrogen atoms in ammonium nitrate

  • Percentage of nitrogen = $\left(\frac{2\times 14}{80}\right)$ × 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