Explain the difference between a molecular formula and a structural formula, and state one piece of information that a structural formula provides which a molecular formula does not.

Cambridge International IGCSE Chemistry (0620) — 3.1 Formulae · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A molecular formula gives the number of each type of atom in one molecule, e.g. C₂H₆O for ethanol. A structural formula shows how those atoms are bonded and arranged, e.g. CH₃CH₂OH, indicating the OH group is attached to the carbon chain.

One piece of information a structural formula provides that a molecular formula does not is the position of functional groups or the connectivity of atoms, such as the location of the OH group in ethanol.

Examiner tips

  • Use the word "molecular" and "structural" clearly; give a concise definition for each.
  • Include a specific example (e.g. ethanol) to illustrate the difference.
  • State the extra information (bonding/position of groups) explicitly.
  • Keep the answer to 4 marks – one sentence each for definition, example, and extra info.

Common mistakes

  • Confusing the molecular formula with the empirical formula.
  • Writing the structural formula incorrectly (e.g. omitting the OH group or misplacing it).
  • Failing to mention the extra information that structural formulas give, such as bond connectivity.

Mark scheme (4 marks)

  1. A molecular formula shows the actual number of each type of atom in one molecule of the compound
  2. A structural formula shows how the atoms are arranged / bonded to each other within the molecule
  3. Structural formula shows the bonds/connections between atoms / how atoms are linked, which molecular formula does not show
  4. Correct example illustrating the difference, e.g. ethanol: molecular formula C₂H₆O / C₂H₅OH; structural formula shows CH₃CH₂OH / shows the OH group separately from the carbon chain

Key terms in this question

molecular formula · structural formula

Related

More Formulae questions

▶ Try answering this question with AI marking (free) →