Paper 1 Q28 — Molecular formula from a skeletal structure

Organic — reading skeletal formulae; counting implicit hydrogens

What the examiners said

“Interpretation of the skeletal formulae was required to deduce that option D did not have the molecular formula C₅H₈O₃, which was the molecular formula of the other three structures. Option C was the most frequent incorrect response.”

Try the question

Three of four skeletal structures share the molecular formula C₅H₈O₃. Reading a skeletal formula, which rule most often causes the miscount that led candidates to the wrong ‘odd one out’?

Examiner report: “Candidates had to read skeletal formulae and count implicit H to find that D was not C₅H₈O₃.”

Explore — count the atoms in a skeletal formula

Toggle the implicit hydrogens on and off, and step through a small structure. Watch the running C / H / O tally and how the degree of unsaturation fixes the hydrogen count.

Structure: A show implicit H

Structure D has only two oxygens — the genuine odd one out. Option C matches C₅H₈O₃ and was wrongly chosen by candidates who miscounted implicit hydrogens or an oxygen.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Chemistry 8873 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org