AQA A-Level Chemistry AS Paper 1, November 2021: Question 16

1 mark · Easy difficulty · Multiple Choice

Identify which compound contains a chlorine atom with an oxidation state of +4

Practise this question

Question

Multiple choice question 16 asking which compound contains a chlorine atom with an oxidation state of +4, with options A (KClO4), B (CCl4), C (ClO2), and D (ClO2F).
Question text

16 Which compound contains a chlorine atom with an oxidation state of +4?

[1 mark]

A KClO4

B CCl4

C ClO2

D ClO2F

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 16 is C, which corresponds to ClO2.

16 C 1 ClO2

How to answer it

Determining Oxidation States

What this question tests

This question assesses your ability to apply the rules of assigning oxidation states to atoms within various neutral compounds containing chlorine, oxygen, potassium, carbon, and fluorine.

Question 16

Multiple Choice [1 Mark]

Which compound contains a chlorine atom with an oxidation state of +4?

✅ Correct Answer: C (ClO₂)

Chlorine dioxide contains chlorine in the +4 oxidation state.

Mark Awarded: 1 / 1 for selecting C

💡 Key Knowledge

  • Oxygen almost always has an oxidation state of -2 (except in peroxides or when bonded to fluorine).
  • Group 1 elements (like Potassium, K) always have an oxidation state of +1 in compounds.
  • Fluorine always has an oxidation state of -1 in compounds.
  • The sum of oxidation states in a neutral compound must equal 0 .

🧠 Exam Technique

Work through the options systematically by setting up simple algebraic equations where the unknown oxidation state of chlorine is represented by x .

❌ Common Errors

Students often misremember the priority rules for electronegative elements like fluorine and oxygen, or confuse the overall charge of a neutral molecule with ion charges.

📐 Step-by-Step Breakdown of All Options

  1. A) KClO₄:
    K = +1, O = -2 (four oxygen atoms = -8).
    (+1) + x + 4(-2) = 0 → 1 + x - 8 = 0 → x = +7 (Incorrect)
  2. B) CCl₄:
    Cl is more electronegative than C, so Cl takes its standard halide oxidation state of -1 (four chlorine atoms = -4).
    x + 4(-1) = 0 → x = +4 for Carbon, but the question asks for chlorine's oxidation state ( -1 ). (Incorrect)
  3. C) ClO₂:
    O = -2 (two oxygen atoms = -4).
    x + 2(-2) = 0 → x - 4 = 0 → x = +4 . (Correct!)
  4. D) ClO₂F:
    O = -2 (two oxygens = -4), F = -1.
    x + 2(-2) + (-1) = 0 → x - 4 - 1 = 0 → x = +5 (Incorrect)

Topics

Physical Chemistry · 3.1.7 Oxidation, Reduction and Redox Equations

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.