WJEC A-Level Chemistry AS Unit 2, June 2025: Question 2

1 mark · Easy difficulty · Short Answer

Name the straight-chain alkane that has the same molecular formula as 2,2,4-trimethylpentane.

Practise this question

Question

Question 2 asking to give the name of the straight chain alkane that has the same molecular formula as 2,2,4-trimethylpentane. A dotted line for the written answer is shown below the question, worth 1 mark.
Question text

2. Give the name of the straight chain alkane that has the same molecular formula as

2,2,4-trimethylpentane. [1]

Mark scheme

Show the mark scheme Mark scheme table row for question 2 showing the single mark awarded for the correct answer 'octane'.

2 octane

How to answer it

Naming Straight Chain Alkane Isomers

📌 What this question tests

This question tests your ability to decode IUPAC systematic names of branched alkanes, count total carbon atoms to deduce molecular formulas, and recall the IUPAC nomenclature of straight-chain (unbranched) alkanes.

Question 2 [1 Mark]

Deducing the Straight Chain Isomer

Identifying the unbranched isomer of 2,2,4-trimethylpentane

✅ Correct Answer

octane

Mark allocation: 1 mark for the correct name octane .

💡 Key Knowledge

  • Stem name: "pentane" indicates a main carbon chain consisting of 5 carbons.
  • Substituents: "trimethyl" indicates three methyl (–CH₃) groups, adding 3 carbons.
  • Total carbons: 5 + 3 = 8 carbons.
  • General formula of alkanes: CnH2n+2, giving a molecular formula of C₈H₁₈.
  • The straight-chain unbranched alkane containing 8 carbon atoms is named octane.

📐 Step-by-Step Breakdown

  1. Read the root chain: pentane = 5 carbons in the longest continuous chain.
  2. Count methyl branches: trimethyl = 3 separate –CH₃ groups = 3 carbons.
  3. Add all carbons together: 5 + 3 = 8 carbons in total.
  4. Check molecular formula: C₈H(2×8)+2 = C₈H₁₈.
  5. Name the straight-chain equivalent: An unbranched 8-carbon alkane is octane .

🧠 Exam Technique & Strategy

  • Look for the word "name": The question specifically demands the name, not the formula. Writing C₈H₁₈ or CH₃(CH₂)₆CH₃ scores 0 marks.
  • Notice "straight chain": This means there are no side branches; simply identify the Greek numerical root for 8 carbons (oct-) with the alkane suffix (-ane).
  • Quick mental check: 2,2,4-trimethylpentane is commercially known as "iso-octane", which is an easy reminder that it has 8 carbons!

❌ Common Errors & Misconceptions

  • Writing the formula instead of the name: Providing C₈H₁₈ is the most frequent blunder. Always re-read whether a formula or a name is requested.
  • Miscounting carbons: Only looking at the parent name "pentane" and writing pentane, or adding only one carbon for methyl and getting hexane.
  • Spelling errors: Writing names like "octene" (an alkene) instead of "octane".

Topics

Organic Chemistry · 2.4 Organic compounds

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.