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Oxidation numbers – practice for Years 8–9

Sample questions on Oxidation numbers with the right answer and a worked solution, from easy to hard. In the Pickles Academy app a child gets new questions of the same kind every time, so they can't just learn the answers by heart.

Subject
Chemistry
Year
Years 8–9
Curriculum area
Compounds, acids and bases

Question types: Find the oxidation number

Rule: Oxidation numbers

The oxidation numbers in a compound add up to 0. Oxygen is usually −2, hydrogen +1.

Work out the unknown one so that the positive numbers balance the negative ones.

Example: N₂O₅: oxygen 5 × (−2) = −10, so the two nitrogens make +10 together and each is +5.

Watch out: Remember the small number: +10 is shared between two nitrogen atoms, so each is +5, not +10.

Sample questions

  1. Question 1 Easy

    What is the oxidation number of aluminium (Al) in Al₂O₃?

    1. A) +3
    2. B) +7
    3. C) +1
    4. D) +4
    5. E) +6
    Show the solution
    Answer: A) +3

    How it works: Al: +3, O: −2; 2 × (+3) + 3 × (−2) = 0.

  2. Question 2 Normal

    What is the oxidation number of nitrogen (N) in NH₃?

    1. A) −3
    2. B) +7
    3. C) +2
    4. D) +4
    5. E) +6
    Show the solution
    Answer: A) −3

    How it works: N: −3, H: +1; (−3) + 3 × (+1) = 0.

  3. Question 3 Hard

    What is the oxidation number of manganese (Mn) in KMnO₄?

    1. A) +7
    2. B) +6
    3. C) +1
    4. D) +3
    5. E) +5
    Show the solution
    Answer: A) +7

    How it works: K: +1, Mn: +7, O: −2; (+1) + (+7) + 4 × (−2) = 0.

Related topics

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