OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 5

1 mark · Easy difficulty · Multiple Choice

Determine the number of hydrogen atoms in one molecule of oxymetazoline given its chemical structure.

Practise this question

Question

Multiple choice question 5 showing the chemical structure of oxymetazoline, featuring an aromatic ring with a hydroxyl group, a tert-butyl group, two methyl groups, and an attached nitrogen-containing ring. Below the structure is the question asking how many H atoms are in one molecule of oxymetazoline, with four options A (23), B (24), C (25), and D (26).
Question text

5 Oxymetazoline, shown below, is used as a decongestant in the treatment of colds.

HO N

HN

Oxymetazoline

How many H atoms are in one molecule of oxymetazoline?

A 23

B 24

C 25

D 26

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 5 is B, with a total of 1 mark.

5 B 1 AO1.2 ALLOW 24

How to answer it

Determining the Number of Hydrogen Atoms from a Skeletal Formula

What this question tests:
This question assesses your ability to interpret skeletal and displayed organic formulae (specifically AO1.2 knowledge) to count individual atoms accurately, accounting for hidden hydrogen atoms on carbon skeletons and functional groups.
Question 5 (Multiple Choice)

Molecular Formula Analysis

Counting Hydrogen Atoms in Oxymetazoline

✅ Correct Answer

Option B: 24

Mark Awarded: 1 / 1 (AO1.2)

💡 Key Knowledge

  • Skeletal Formula Rules: Terminals and intersections represent carbon atoms with implied hydrogen atoms bonded to fill remaining valencies (making 4 bonds total for carbon).
  • Explicit Atoms: Heteroatoms like O and N, and the hydrogens directly attached to them (e.g., -OH and -NH- ), are written out explicitly.

🧠 Exam Technique

Systematically break down the molecule into distinct structural fragments to avoid double-counting or missing hidden hydrogens:

  1. Count explicit hydrogens shown on heteroatoms.
  2. Count methyl groups ( -CH₃ ).
  3. Count alkyl chain/ring hydrogens.
  4. Count aromatic ring substituents/hydrogens.

❌ Common Errors

  • Forgetting hidden carbons: Miscounting vertices on the imidazoline ring or the tertiary-butyl group.
  • Aromatic ring traps: Forgetting that substituted carbons on a benzene ring have zero hydrogens attached, while unsubstituted carbons have exactly one.

📐 Step-by-Step Breakdown for Oxymetazoline (C₁₆H₂₄N₂O)

  • Step 1 (Phenol OH): 1 hydrogen from the -OH group.
  • Step 2 (Aromatic Ring): Benzene ring has 1 hydrogen at the bottom position (the other 5 positions are substituted by methyl, tertiary-butyl, OH, and a CH₂ group). Total = 1 H.
  • Step 3 (Methyl groups): Two aromatic ring methyl groups ( 2 × -CH₃ ) = 6 H.
  • Step 4 (Tertiary-butyl group): One -C(CH₃)₃ group attached to the ring = 9 H.
  • Step 5 (Methylene bridge): The -CH₂- group linking the benzene ring to the imidazoline ring = 2 H.
  • Step 6 (Imidazoline ring): Contains an -NH- group (1 H) and two ring -CH₂- groups ( 2 × 2 = 4 H). Total = 5 H.
  • Total Calculation: 1 + 1 + 6 + 9 + 2 + 5 = 24 Hydrogen atoms.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.