AQA A-Level Chemistry Paper 3, 2021: Question 25

1 mark · Medium difficulty · Multiple Choice

This question checks your understanding of how molecular structure influences the amount of oxygen needed for complete combustion. You are asked to compare four organic compounds and determine which one requires the most oxygen per mole, based on the number and types of atoms present in each molecule.

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Question

AQA A-Level Chemistry Paper 3, 2021: Question 25
Question text

25 Which compound needs the greatest amount of oxygen for the complete combustion

of 1 mol of the compound?

[1 mark]

A ethanal

B ethanol

C ethane-1,2-diol

D methanol

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 25

25 B 1 ethanol

How to answer it

Multiple Choice Explained: Greatest Oxygen Requirement for Combustion

Correct Answer: B – Ethanol

Why? The complete combustion of ethanol (C2H5OH) requires more oxygen molecules than the other compounds listed.

The balanced equation is:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O
→ 3 moles of O2 per mole of ethanol.

Why A is incorrect:

Ethanal (CH3CHO) combustion:
CH3CHO + 2.5 O2 → 2 CO2 + 2 H2O
→ Requires 2.5 moles of oxygen, less than ethanol.

Why C is incorrect:

Ethane-1,2-diol (C2H6O2) combustion:
C2H6O2 + 2.5 O2 → 2 CO2 + 3 H2O
→ Also only requires 2.5 moles of oxygen per mole.

Why D is incorrect:

Methanol (CH3OH) combustion:
CH3OH + 1.5 O2 → CO2 + 2 H2O
→ Requires only 1.5 moles of oxygen per mole – the least of all.

Topics

Physical Chemistry · 3.1.2 Amount of Substance

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.