Glucose has the formula C₆H₁₂O₆. State what is meant by the term relative formula mass and describe the steps you would take to calculate the relative formula mass of glucose. Use the relative atomic masses: H = 1, C = 12, O = 16.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.4 Relative formula mass · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Glucose is a sugar found in many fruits. It has the molecular formula C₆H₁₂O₆.

Model answer (4 marks)

Relative formula mass is the sum of the relative atomic masses of all the atoms in the formula of the compound.

To calculate the relative formula mass of glucose (C₆H₁₂O₆) use the given relative atomic masses:
1. Multiply the relative atomic mass of each element by the number of atoms of that element in the formula:
• C: 12 × 6 = 72
• H: 1 × 12 = 12
• O: 16 × 6 = 96
2. Add the values for all elements: 72 + 12 + 96 = 180

Therefore, the relative formula mass of glucose is 180.

Examiner tips

  • Define the term first, then show the multiplication for each element, finally add the results and state the final value.
  • Use the exact wording from the mark scheme: ‘sum of the relative atomic masses’ and ‘multiply … by the number of atoms’.

Common mistakes

  • Using the wrong atomic masses (e.g. 1.008 for H instead of 1), "forgetting to multiply by the number of atoms", or "adding incorrectly, giving 179 or 181”.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in the formula (of the compound).
  2. Multiply the relative atomic mass of each element by the number of atoms of that element in the formula (e.g. C: 12 × 6 = 72; H: 1 × 12 = 12; O: 16 × 6 = 96).
  3. Add together the values for all elements (72 + 12 + 96).
  4. Relative formula mass of glucose = 180.

Key terms in this question

relative formula mass · relative atomic mass · formula

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