AQA A-Level Chemistry Paper 3, 2021: Question 27
1 mark · Easy difficulty · Multiple Choice
This question checks your knowledge of general formulae for different families of organic compounds. It requires you to recognise the characteristic molecular structure and number of atoms for alcohols, aldehydes, esters, and primary amines in a homologous series of non-cyclic molecules.
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Question text
27 Which is the correct general formula for non-cyclic compounds in the
homologous series?
[1 mark]
A alcohols CnH2n+2O
B aldehydes CnH2n+1O
C esters CnH2n+1O2
D primary amines CnH2n+2N
Mark scheme
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27 A 1 alcohols CnH2n+2O
How to answer it
Multiple Choice Explained: General Formula of Homologous Series
Correct Answer: A – Alcohols CnH2n+2O
Why? Alcohols follow the general formula CnH2n+2O for non-cyclic, saturated structures. The –OH functional group replaces one hydrogen atom in the alkane chain, so the carbon and oxygen count stays consistent with this formula.
Why B is incorrect:
Aldehydes have the formula CnH2nO, not CnH2n+1O. The extra hydrogen makes this formula invalid for simple aldehydes.
Why C is incorrect:
Esters have the general formula CnH2nO2, not CnH2n+1O2. The incorrect hydrogen count makes this answer invalid.
Why D is incorrect:
Primary amines follow the formula CnH2n+3N (not CnH2n+2N) because the –NH2 group contributes one nitrogen and two hydrogens, increasing the hydrogen count accordingly.
Topics
Organic Chemistry · 3.3.1 Introduction to Organic Chemistry
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.