OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 3
1 mark · Easy difficulty · Multiple Choice
Identify the general formula of the alkynes homologous series given propyne as an example.
Practise this questionQuestion
Question text
3 Propyne, CH3C≡CH, is a member of the alkynes homologous series with the C≡C functional group.
What is the general formula of the alkynes?
A CnH2n–4
B CnH2n–2
C CnH2n
D CnH2n+2
Your answer
[1]
Mark scheme
Show the mark scheme
3 B 1 AO2.1
How to answer it
General Formula of Alkynes
What this question tests
This question assesses your understanding of homologous series in organic chemistry, specifically your ability to deduce or recall the general formula for unsaturated hydrocarbons containing a triple carbon-carbon bond (alkynes) by analysing molecular structures.
Determining the General Formula
✅ Correct Answer
Option B: CnH₂ₙ₋₂
Alkynes contain one triple bond. Each triple bond introduces an additional ring or degree of unsaturation compared to alkenes, removing two more hydrogen atoms.
💡 Key Knowledge
- Alkanes (saturated): CnH₂ₙ₊₂
- Alkenes (one double bond) / Cycloalkanes: CnH₂ₙ
- Alkynes (one triple bond) / Dienes: CnH₂ₙ₋₂
🧠 Exam Technique
If you ever forget a general formula under exam pressure, plug in a real example from the stem! Propyne is given as CH₃C≡CH , which has 3 carbons ( n = 3 ) and 4 hydrogens ( H = 4 ). Test the options:
- A: 2(3) - 4 = 2
- B: 2(3) - 2 = 4 (Matches!)
- C: 2(3) = 6
❌ Common Errors
Students often confuse the degree of unsaturation for double vs. triple bonds. A common mistake is selecting CnH₂ₙ (confusing alkynes with alkenes) or guessing CnH₂ₙ₋₄ by overestimating the hydrogen loss.
Topics
Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons
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.