OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 12

1 mark · Medium difficulty · Multiple Choice

Determine the molecular formula of the given steroid molecule containing an aromatic ring and two hydroxyl groups.

Practise this question

Question

Question 12 asks for the molecular formula of a steroid molecule shown as a skeletal formula containing a benzene ring with an OH group, fused saturated carbon rings, a methyl group, and another OH group. Below the structure are four multiple-choice options: A, C18H24O2; B, C18H26O2; C, C18H28O2; and D, C18H30O2, with a box for the student's answer.
Question text

12 What is the molecular formula of the steroid molecule below?

OH

HO

A C18H24O2

B C18H26O2

C C18H28O2

D C18H30O2

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer for question 12 is option A, worth 1 mark.

12 A 1 2.2

How to answer it

Determining Molecular Formula from a Skeletal Structure

📌 What this question tests

This question assesses your ability to interpret complex skeletal formulas, apply the rules of carbon valency (tetravalency) to deduce hidden hydrogen atoms, account for functional groups (such as hydroxyl groups, OH ), and recognize structural features like aromatic benzene rings within cyclic systems.

Question 12: Molecular Formula Analysis

Exam Breakdown & Solutions

✅ Correct Answer

A ( C₁₈H₂₄O₂ )

Mark Awarded: 1 / 1

💡 Key Knowledge

  • Carbon Vertices: Every corner/vertex in the skeletal formula represents a carbon atom bonded to sufficient hydrogens to make 4 bonds in total.
  • Benzene Ring: The ring with the circle represents a benzene ring ( C₆H₅– when attached, or C₆H₆ base).
  • Oxygen Atoms: Two -OH groups are explicitly shown on the structure, meaning exactly 2 oxygen atoms must be present in the final formula.

🧠 Exam Technique

  • Notice that all multiple-choice options share the same number of carbons ( C₁₈ ) and oxygens ( O₂ ). Do not waste time counting carbons! Focus exclusively on counting and verifying the hydrogen atoms.
  • Break the complex polycyclic molecule down into recognizable fragments (e.g., benzene ring, alkyl rings, methyl group, alcohol groups) to avoid double-counting or missing hydrogens.

❌ Common Errors

  • Forgetting Junction Hydrogens: Forgetting that carbons at ring junctions (where three or four carbon-carbon bonds meet) only have 1 hydrogen atom (or 0 if fully substituted).
  • Aromatic Ring Confusion: Treating the benzene ring carbons as standard saturated alkane chain carbons ( -CH₂- ), leading to a massive overcount of hydrogen atoms.
  • Alcohol Hydrogens: Forgetting to include the hydrogen atom from the -OH hydroxyl groups in the final hydrogen tally.

📐 Step-by-Step Hydrogen Counting Breakdown

  1. Aromatic Ring with one -OH substituent: A standard phenol-type ring has 4 remaining ring hydrogens + 1 hydrogen from the -OH group = 5 hydrogens.
  2. Aliphatic Rings (middle and top-right saturated carbon rings): Carefully count carbons and subtract bonds already drawn to find attached hydrogens (remembering junction carbons only have 1 H). Tallying these saturated ring carbons and the projecting methyl group ( -CH₃ ) yields 18 hydrogens.
  3. Top-right Hydroxyl group: The second -OH group contributes 1 hydrogen.
  4. Total Hydrogen Sum: 5 (aromatic system) + 18 (aliphatic framework/methyl) + 1 ( -OH ) = H₂₄ . Combined with C₁₈ and O₂ , this confirms option A.

Topics

Module 6: Organic chemistry and analysis · Module 4: Core organic chemistry · 6.1 Aromatic compounds, carbonyls and acids · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.