OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 17

✅ Correct Answer: A ( C₁₀H₈ )

Naphthalene consists of two fused benzene rings. Counting the carbon vertices and accounting for the shared central carbons yields 10 carbons and 8 hydrogens.

💡 Key Knowledge

• Skeletal Formula Rules: Each vertex represents a carbon atom.
• Fused Rings: Two carbons are shared at the bridgehead position between the rings and do not bond to hydrogen atoms.
• Valency: Carbon must form 4 covalent bonds. Unoccupied bonds at each vertex correspond to hydrogen atoms attached to carbons.

🧠 Exam Technique

Don't guess based on standard benzene derivatives. Systematically count:

1. Count total outer and inner ring vertices to find carbons ( 10 ).
2. Identify that the 2 bridgehead carbons have 0 hydrogens attached (all 4 bonds are already used within the ring framework).
3. Count peripheral carbons with available bonding space ( 8 carbons each bonded to 1 hydrogen).

❌ Common Errors

• Option B ( C₁₀H₁₀ ): Assuming every carbon atom around the perimeter holds a hydrogen atom, forgetting that fused rings share carbons.
• Counting loops instead of atoms: Misinterpreting aromatic rings as simple saturated alkane ring systems.