AQA A-Level Chemistry Paper 1, 2022: Question 5

✅ Correct Answer

Box 2: "It forms a solution with pH = 14 at 25 °C"

💡 Key Knowledge

• Mg(OH)₂ is sparingly soluble in water. Due to low [OH⁻] in solution, it only reaches a pH of approximately 9 to 10.
• It is safely used in medicine (Milk of Magnesia) to neutralize excess stomach acid (HCl) without burning esophagus tissues.
• The molecular formula is Mg(OH)₂, giving an empirical formula of H₂MgO₂ (ratio 1 Mg : 2 O : 2 H).

❌ Common Errors

• Misreading the prompt: Overlooking the word "not" and ticking one of the correct statements.
• Confusing the sparingly soluble Mg(OH)₂ with group 1 hydroxides (like NaOH), which are fully soluble and achieve pH 14 at 1.0 mol dm⁻³.

✅ Balanced Chemical Equation

Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂O

💡 Chemical Insight

• Al₂O₃ is amphoteric: it reacts with bases to form aluminates ([Al(OH)₄]⁻) and with acids to form aluminium salts and water.
• Aluminium forms Al³⁺ ions and sulfate is SO₄²⁻, meaning aluminium sulfate must be Al₂(SO₄)₃.

❌ Common Errors

• Writing incorrect salt formulas such as AlSO₄ or Al(SO₄)₂.
• Failing to balance sulfuric acid molecules: 3 SO₄²⁻ ions require 3 H₂SO₄, yielding 3 H₂O.

✅ Accepted Methods & Observations

Any ONE of the following valid reagent/test routes:

• Method 1: Acidified Barium Chloride [BaCl₂ / HCl]
  • SO₂(aq): No visible change (or stays colourless)
  • SO₃(aq): White precipitate formed (BaSO₄)
• Method 2: pH Meter
  • SO₂(aq): Higher pH / pH 2–3 (weak acid: H₂SO₃)
  • SO₃(aq): Lower pH / pH 0–1 (strong acid: H₂SO₄)
• Method 3: Universal Indicator
  • SO₂(aq): Orange / Yellow
  • SO₃(aq): Red
• Method 4: Named metal carbonate (or Mg / Ca / Zn)
  • SO₂(aq): Slower effervescence
  • SO₃(aq): Faster effervescence
• Method 5: Acidified KMnO₄ or K₂Cr₂O₇
  • SO₂(aq): Purple to colourless (KMnO₄) or orange to green (K₂Cr₂O₇)
  • SO₃(aq): No visible change

🧠 Exam Technique

• When dissolved in water:
  • SO₂ + H₂O → H₂SO₃ (sulfurous acid, weak)
  • SO₃ + H₂O → H₂SO₄ (sulfuric acid, strong)
• Barium chloride is by far the most definitive test because SO₃(aq) contains free SO₄²⁻ ions which immediately precipitate as BaSO₄(s).
• If testing acidity with indicator, ensure you quote a comparative difference (e.g., orange vs red). Simply writing "turns red" for both loses both observation marks.

✅ Formula of the Species

³¹P₄⁺   (or P₄⁺)

📐 Step-by-Step Deduction

1. Phosphorus exists naturally as white phosphorus molecules consisting of 4 atoms (P₄).
2. The atomic mass of the sole stable isotope of phosphorus is 31:
   124 ÷ 31 = 4 .
3. In mass spectrometry, detected species must be positive ions formed by electron impact ionization:
   P₄ + e⁻ → P₄⁺ + 2e⁻ .

❌ Common Errors

• Forgetting the positive charge: Writing just P₄ or ³¹P₄ scores 0 marks. Mass spectrometers only detect charged ions!
• Omitting the isotope number ³¹ when full credit requires identification of the isotope giving m/z = 124.

✅ Balanced Chemical Equation

P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O

💡 Key Knowledge

• Phosphorus(V) oxide (P₄O₁₀) is a strongly acidic oxide.
• It reacts with alkalis to form phosphate(V) salts (PO₄³⁻) and water.
• Ensure the formula is written as P₄O₁₀ (the molecular formula), not the empirical formula P₂O₅, unless specified.

✅ Displayed Formula

Reaction: P₄O₁₀ + 6H₂O → 4H₃PO₄ (phosphoric(V) acid)

❌ Major Mark Traps

• Writing —OH instead of —O—H . A displayed formula requires every single covalent bond to be drawn explicitly as a line.
• Drawing 5 single bonds to OH groups (P(OH)₅ does not exist in this context).

🧠 Level 3 Marking Structure (5–6 Marks)

Examiners mark this using 3 distinct logical stages. To get 6/6, all 3 stages must be fully covered:

• Stage 1: Types of Structures
  • NaCl is a giant ionic lattice.
  • Cl₂ and HCl are both simple molecular substances.
• Stage 2: Specific Forces Broken
  • In NaCl: Strong electrostatic attractions between Na⁺ and Cl⁻ ions.
  • In Cl₂: Van der Waals forces between Cl₂ molecules.
  • In HCl: Permanent dipole-dipole forces (and vdW) between HCl molecules.
• Stage 3: Comparing Strengths to Data
  • Ionic vs Molecular: Ionic bonds in NaCl are much stronger than the intermolecular forces in Cl₂ and HCl, requiring far more energy to overcome (hence 1074 K).
  • Cl₂ vs HCl: Cl₂ has more electrons than HCl (34 e⁻ in Cl₂ vs 18 e⁻ in HCl).
  • Therefore, van der Waals forces between Cl₂ molecules are stronger than the dipole-dipole & vdW forces between HCl molecules, giving Cl₂ a higher melting point (172 K vs 158 K).

❌ Critical Misconceptions in this 6-Marker

• Breaking covalent bonds: Saying "covalent bonds in Cl₂ or HCl are broken when melting". Never say this! Only weak intermolecular forces break.
• Assuming dipole-dipole is always stronger: Many students write: "HCl has dipole-dipole so it must have a higher melting point than Cl₂." Look at the data! Cl₂ is higher (172 K > 158 K). The larger electron cloud in Cl₂ creates van der Waals forces that outweigh HCl's permanent dipoles.
• Vague ion terminology: Saying "attraction between molecules in NaCl" or failing to specify that attractions are between oppositely charged ions (Na⁺ and Cl⁻).
• Contradictions negate marks: E.g., correctly stating "intermolecular forces" but later referring to "breaking H—Cl bonds".