Explain how the empirical formula of a compound can be determined from its percentage composition by mass, and outline why the mole concept is essential in this process.

IB DP Chemistry Standard Level (2023 syllabus) — S1.4 Counting particles by mass: The mole · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Convert the mass percentages to moles by dividing each percentage by the element’s atomic mass.
2. Divide each mole value by the smallest mole value to obtain the simplest mole ratios.
3. Express these ratios as whole numbers to give the empirical formula.
4. The mole concept is essential because it links the macroscopic mass data to the microscopic number of atoms, allowing the mass ratios to be interpreted as atomic ratios.

Examiner tips

  • Use the exact wording ‘convert to moles’, ‘divide by the smallest mole value’, ‘whole‑number ratios’
  • Show the conversion step for at least one element
  • Explain the role of the mole concept in one sentence

Common mistakes

  • Failing to divide by the smallest mole value
  • Using incorrect atomic masses
  • Not recognising that the mole concept links mass to atom count

Mark scheme (4 marks)

  1. Percentage by mass values are converted to an amount in moles by dividing each by the respective molar mass (atomic mass).
  2. The mole ratios are obtained by dividing all mole values by the smallest mole value, giving a ratio of atoms.
  3. These whole-number ratios give the simplest ratio of atoms of each element, which defines the empirical formula.
  4. The mole concept is essential because it links mass (macroscopic, measurable) to the number of atoms (sub-microscopic), allowing mass ratios to be interpreted as atomic ratios.

Key terms in this question

empirical formula · percentage composition by mass · mole

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