AQA A-Level Chemistry Paper 3, 2021: Question 35

1 mark · Easy difficulty · Multiple Choice

This question tests your understanding of zwitterions—molecules that have both a positive and a negative charge but are overall neutral. You're asked to identify which structure correctly shows a zwitterion form of an amino acid. Zwitterions typically form in aqueous solution where the amino group becomes protonated and the carboxylic acid group loses a proton.

Practise this question

Question

AQA A-Level Chemistry Paper 3, 2021: Question 35
Question text

35 Which is the structure of a zwitterion of an amino acid?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 35

35 D 1

How to answer it

Multiple Choice Explained: Zwitterion of an Amino Acid

Correct Answer: D

This is the structure of a zwitterion: it contains a positively charged ammonium group (NH₃⁺) and a negatively charged carboxylate group (COO⁻). The R group here is –CH₂SH (from cysteine), but the presence of both charges on the main chain confirms it's a zwitterion.

Why A is incorrect:

This molecule has two positively charged NH₃⁺ groups and one COO⁻, resulting in an overall +1 charge. A zwitterion must be overall neutral.

Why B is incorrect:

This structure contains two COO⁻ groups and one NH₃⁺, making it overall negatively charged—not a zwitterion.

Why C is incorrect:

This contains a neutral –OH₂⁺ group instead of the correct zwitterionic structure. Additionally, the side chain seems unrealistic for an amino acid and doesn't represent a standard amino acid.

Topics

Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA

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.