AQA A-Level Chemistry AS Paper 1, June 2025: Question 8

1 mark · Easy difficulty · Multiple Choice

Identify the correct chemical formula of gallium sulfate(VI).

Practise this question

Question

Question 8 asks: 'What is the formula of gallium sulfate(VI)?' followed by four multiple-choice options with selection boxes: A: Ga2(SO4)3, B: Ga3(SO4)2, C: Ga2(SO3)3, and D: GaSO4. It is worth 1 mark.
Question text

08 What is the formula of gallium sulfate(VI)?

[1 mark]

A Ga2(SO4)3

B Ga3(SO4)2

C Ga2(SO3)3

D GaSO4

Mark scheme

Show the mark scheme Mark scheme table shows for question 8: Marking Guidance is 'A', Mark is '1 (AO1)', and Comments list 'Ga2(SO4)3'.

Question Marking Guidance Mark Comments

8 A 1 (AO1) Ga2(SO4)3

How to answer it

Deducing the Chemical Formula of Gallium Sulfate(VI)

📌 What this question tests

This multiple-choice question assesses your fundamental knowledge of inorganic nomenclature, ionic charges, and balancing charges to construct neutral ionic formulae:

  • Deducing the charge of main-group metal cations using the Periodic Table (Periodic trends: Group 13/Group 3).
  • Recognising the formula and charge of the sulfate(VI) oxyanion ( SO₄²⁻ ).
  • Applying the crossover/ratio method to balance positive and negative charges to achieve overall electrical neutrality.
Question 08 • Multiple Choice

Full Question Analysis

Identifying the correct option for gallium sulfate(VI) [1 Mark]

✅ Correct Answer: Option A

Formula: Ga₂(SO₄)₃

Gallium forms Ga³⁺ ions and sulfate(VI) is SO₄²⁻ . Two Ga³⁺ ions (+6) balance three SO₄²⁻ ions (-6).

📐 Step-by-Step Formula Deduction

  1. Find cation charge: Gallium (Ga) is in Group 13 (Group 3). It loses 3 valence electrons to form Ga³⁺.
  2. Find anion formula & charge: Sulfate(VI) contains sulfur with oxidation state +6. Combined with four oxide ions (-2 each):
    +6 + 4(-2) = -2, giving SO₄²⁻.
  3. Balance the charges: Lowest common multiple of 3 and 2 is 6.
    • Need 2 × (+3) = +6 → 2 × Ga³⁺
    • Need 3 × (-2) = -6 → 3 × SO₄²⁻
  4. Combine using brackets: Ga₂(SO₄)₃.

💡 Key Knowledge

  • Group 13 Metals: Elements like Al and Ga predominantly form 3+ oxidation states in simple ionic compounds.
  • Sulfate(VI) vs Sulfate(IV):
    • Sulfate(VI) is SO₄²⁻ (sulfur oxidation state = +6).
    • Sulfate(IV) or sulfite is SO₃²⁻ (sulfur oxidation state = +4).
  • Polyatomic Ions: Always enclose compound ions in brackets when more than one is needed in the formula unit (e.g., (SO₄)₃ ).

❌ Common Errors & Distractor Traps

  • Option B ( Ga₃(SO₄)₂ ): Inverting the charges when doing the "swap and drop" method (assigning Ga a 2+ charge and sulfate a 3- charge).
  • Option C ( Ga₂(SO₃)₃ ): Confusing sulfate(VI) with sulfate(IV) / sulfite ( SO₃²⁻ ).
  • Option D ( GaSO₄ ): Assuming gallium behaves like Group 2 metals (forming a 2+ ion) or simply writing a 1:1 ratio.

🧠 Exam Technique & Speed Tips

In Section B multiple-choice questions, speed and accuracy are vital:

  • Check the Periodic Table immediately: Find Ga in the data booklet. It sits directly under Aluminium (Al). Since aluminium sulfate is familiar to you as Al₂(SO₄)₃ , gallium sulfate must follow the exact same stoichiometry: Ga₂(SO₄)₃ .
  • Eliminate Roman numeral confusion: Remember that the "(VI)" in sulfate(VI) is the oxidation state of sulfur, not the charge on the anion. The sulfate ion charge remains 2-.
Mark Scheme Breakdown: 1 mark awarded for selecting A ( Ga₂(SO₄)₃ ) under assessment objective AO1 (Recall and Application of Knowledge).

Topics

Physical Chemistry · 3.1.2 Amount of Substance · 3.1.3 Bonding · 3.1.7 Oxidation, Reduction and Redox Equations

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.