AQA A-Level Chemistry Paper 3, June 2025: Question 31

1 mark · Medium difficulty · Multiple Choice

Identify the shapes around the central atom in the acid molecules formed when phosphorus(V) oxide and sulfur dioxide react with water.

Practise this question

Question

Question 31 asks: 'The oxides P4O10 and SO2 both react with water to form acid molecules. Which row gives the correct shape around the central atom for each acid molecule that is formed?' A table is given with four rows: A (pyramidal, pyramidal), B (pyramidal, tetrahedral), C (tetrahedral, pyramidal), and D (tetrahedral, tetrahedral), corresponding to the molecules formed from P4O10 and SO2 respectively.
Question text

31 The oxides P4O10 and SO2 both react with water to form acid molecules.

Which row gives the correct shape around the central atom for each acid molecule

that is formed?

[1 mark]

Molecule formed from P4O10 Molecule formed from SO2

A pyramidal pyramidal

B pyramidal tetrahedral

C tetrahedral pyramidal

D tetrahedral tetrahedral

Mark scheme

Show the mark scheme Mark scheme for question 31 showing the correct answer is C with 1 mark for AO1, corresponding to tetrahedral for the molecule formed from P4O10 and pyramidal for the molecule formed from SO2.

31 C 1 (AO1)

C tetrahedral pyramidal

How to answer it

Molecular Shapes of Acids Formed by Period 3 Oxides

📋 What this question tests

This question links two fundamental A-Level Chemistry topics:

  • Period 3 Periodicity (Inorganic Chemistry): Knowing the correct chemical formulas and identities of the acids formed when non-metal oxides (P₄O₁₀ and SO₂) react with water.
  • VSEPR Theory & Molecular Shapes (Physical Chemistry): Deducing the 3D arrangement and molecular shape around central atoms using electron pair repulsion theory (bonding pairs vs lone pairs).

Question 31 Analysis (1 Mark Multiple Choice)

Identifying the correct shape around the central atom for each formed acid

✅ Correct Answer

Option C: tetrahedral, pyramidal

  • Acid from P₄O₁₀: Phosphoric(V) acid, H₃PO₄ → tetrahedral
  • Acid from SO₂: Sulfuric(IV) acid (sulfurous acid), H₂SO₃ → pyramidal

🧠 Exam Technique & Strategy

Step 1: Write down the acid formulas
Do not guess shapes from the oxides directly! First identify the acid produced in aqueous solution:
• P₄O₁₀ + 6H₂O → 4H₃PO₄
• SO₂ + H₂O → H₂SO₃

Step 2: Count valence electron pairs around the central atom
Remember that double bonds count as single bonding regions (centres of electron density) when determining molecular geometry under VSEPR theory.

📐 Step-by-Step VSEPR Deduction

1. Central Phosphorus in H₃PO₄:
  • Phosphorus has 5 valence electrons.
  • Structure: O=P(OH)₃
  • Bonds formed: 1 double bond to =O (shares 2 electrons) and 3 single bonds to -OH groups (shares 3 electrons). All 5 valence electrons are bonded.
  • Regions of electron density: 4 bonding groups, 0 lone pairs.
  • Shape: Tetrahedral (bond angles ≈ 109.5°).
2. Central Sulfur in H₂SO₃:
  • Sulfur has 6 valence electrons.
  • Structure: O=S(OH)₂
  • Bonds formed: 1 double bond to =O (shares 2 electrons) and 2 single bonds to -OH groups (shares 2 electrons). Total bonded electrons = 4.
  • Unbonded electrons: 6 − 4 = 2 electrons = 1 lone pair.
  • Regions of electron density: 3 bonding groups, 1 lone pair (4 electron pairs total, based on a tetrahedral arrangement).
  • Shape: Trigonal pyramidal (bond angle ≈ 107°).

💡 Key Knowledge: Period 3 Oxides

  • P₄O₁₀: Vigorous reaction with water:
    P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
    Forms a strong-to-moderate acidic solution (pH ≈ 0 to 1).
  • SO₂: Dissolves and reacts reversibly:
    SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)
    Forms a weak acidic solution (pH ≈ 2 to 3).
  • Contrast with SO₃:
    SO₃(g) + H₂O(l) → H₂SO₄(aq)
    Sulfuric(VI) acid has 4 bonding groups around S and 0 lone pairs, giving a tetrahedral shape!

❌ Common Errors & Misconceptions

  • Confusing SO₂ with SO₃: The most common student error is assuming the acid formed from SO₂ is sulfuric(VI) acid ( H₂SO₄ ), which is tetrahedral. This led many candidates to incorrectly choose Option D. Remember: SO₂ forms sulfurous acid ( H₂SO₃ ), preserving the +4 oxidation state of sulfur!
  • Determining the shape of the oxide instead of the acid: Rushing the question and looking at the shape around S in SO₂ (which is bent/V-shaped) rather than the acid formed.
  • Forgetting the lone pair on sulfur in H₂SO₃: Sulfur has 6 valence electrons; with one S=O and two S-OH bonds, there is still one non-bonding lone pair remaining, turning a planar shape into a pyramidal one.
Mark Scheme Breakdown: 1 Mark total (AO1 recall & application). Awarded strictly for selecting row C.

Topics

Physical Chemistry · Inorganic Chemistry · 3.1.3 Bonding · 3.2.4 Properties of Period 3 Elements and Their Oxides

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.