AQA A-Level Chemistry Paper 3, June 2025: Question 26

AQA A-Level Chemistry • Paper 1 • Inorganic Chemistry

Reactions and Stoichiometry of Period 3 Oxides

What this question tests

This question evaluates your recall and application of Period 3 oxide equations (Topic 3.2.4). Specifically, it assesses your ability to balance acid–base and hydration reactions of Period 3 oxides (Na₂O, MgO, Al₂O₃, and P₄O₁₀) and determine the exact reacting molar ratios.

Question 26 Analysis

Multiple Choice: Evaluating Options A to D

✅ Correct Answer: C

A solution of hydrochloric acid containing 6 mol of HCl reacts completely with 1 mol of Al₂O₃

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Aluminium oxide acts as a basic oxide when reacting with strong acids. One mole of Al₂O₃ (containing 3 oxide ions, O²⁻) precisely requires 6 moles of H⁺ ions (from 6 mol HCl) to form 3 moles of water and soluble aluminium chloride.

Award: 1 Mark (AO2) for selecting option C.

📐 Step-by-Step Breakdown of Options

Option A: Na₂O + H₂O → 2NaOH
Ratio is 1 : 1. Thus, 1 mol of Na₂O requires 1.0 mol of H₂O, not 0.5 mol.
Option B: MgO + 2HCl → MgCl₂ + H₂O
Ratio is 1 : 2. Adding 1 mol HCl to 1 mol MgO leaves 0.5 mol of basic MgO unreacted. The resulting mixture is weakly alkaline (pH ~9–10), not pH 7.
Option C: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Ratio is 1 : 6. Exactly 6 mol HCl reacts with 1 mol Al₂O₃.
Option D: P₄O₁₀ + 6H₂O → 4H₃PO₄
Ratio is 1 : 6. 1 mol of P₄O₁₀ requires 6 mol of H₂O, not 4 mol.

💡 Key Knowledge: Period 3 Oxides Reference

Na₂O (basic): Dissolves vigorously in water to give strongly alkaline NaOH solution (pH 13–14).
MgO (basic): Sparingly soluble in water forming Mg(OH)₂ (pH ~9). Dissolves readily in acids (1 : 2 ratio).
Al₂O₃ (amphoteric): Insoluble in water. Reacts with acids as a base:
Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O
Reacts with hot concentrated alkalis as an acid:
Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]
P₄O₁₀ (acidic): Reacts violently with water to form phosphoric(V) acid (pH 0–1):
P₄O₁₀ + 6H₂O → 4H₃PO₄

❌ Common Misconceptions & Traps

The "2 NaOH" Confusion (Option A): Students see that 2 moles of NaOH are formed and mistakenly halve the required water to 0.5 mol, forgetting that 1 mole of O comes from water and 1 from Na₂O.
Assuming 1:1 Acid–Base Neutralisation (Option B): Students overlook the 2+ charge on Mg²⁺, wrongly assuming 1 mol of any base neutralises 1 mol of HCl to give pH 7.
Phosphoric Acid Formula Confusion (Option D): Confusing the stoichiometric coefficient of the product (4H₃PO₄) with the coefficient of water required (6H₂O).

🧠 Exam Technique for Section B Multiple Choice

In multiple choice questions testing reaction stoichiometry, do not try to guess or balance mentally:

Jot down the balanced equation immediately on your question paper beside each option.
Check the stoichiometric mole ratio directly against the numbers stated in the option.
Identify limiting reactants: If an option mentions resulting pH (like Option B), check whether acid or base is in excess.