AQA A-Level Chemistry AS Paper 1, June 2023: Question 2

✅ Correct Answer

• M1: (Giant) lattice of Mg²⁺ cations / (giant) lattice of magnesium atoms.
• M2: Electrostatic attraction between cations / Mg²⁺ ions / nuclei and delocalised electrons.

🧠 Exam Technique

Always state both the type of particle and the arrangement when describing structure:

• Structure = giant metallic lattice.
• Bonding = electrostatic force between positive metal ions and delocalised electrons.

❌ Common Errors & Misconceptions

• Stating "covalent bonds" or "intermolecular forces" instantly forfeits M1.
• Forgetting the word delocalised when referencing electrons loses M2.
• Saying "attraction between magnesium atoms and electrons" (atoms are neutral; the attraction is to cations/nuclei).

💡 Key Knowledge

Magnesium has metallic bonding: Mg atoms each donate 2 valence electrons into a delocalised "sea" of electrons, forming an array of closely packed Mg²⁺ ions held tightly by electrostatic forces.

✅ Correct Answer

• Trend (M1): Increases.
• Reason (M2): The number of electron shells (or electron energy levels) increases.

❌ Common Errors

• Writing "shielding increases" as the primary reason. While shielding does increase, AQA explicitly requires mentioning the increase in shells / main energy levels for the mark.
• Confusing group trends with period trends (e.g. arguing that nuclear charge increases so radius decreases).

✅ Correct Answer

Equation:

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Observations (any two):

• Bright white flame / white light
• White / grey solid / ash / powder (smoke allowed)

💡 Key Knowledge: Steam vs Cold Water

• With steam: Mg burns vigorously to form magnesium oxide (MgO) and hydrogen (H₂). State symbol for steam is (g) !
• With cold water: Reaction is very slow; forms magnesium hydroxide: Mg(s) + 2H₂O(l) → Mg(OH)₂(aq/s) + H₂(g) .

❌ Common Errors

• Writing H₂O(l) instead of H₂O(g) — state symbols are explicitly stated as essential.
• Producing Mg(OH)₂ instead of MgO .
• Calling the solid product a "precipitate" — precipitates only form in aqueous solution, not when a gas reacts with a solid!
• Vague observations like "fumes", "effervescence", or "black solid".

🧠 Exam Technique

Always inspect the question prompt: when it specifies "including state symbols", full equation marks depend strictly on correct states: (s) , (g) .

✅ Correct Answer

• Formula: BaSO₄
• Use: Barium meal / X-ray imaging (of digestive system / gut / internal organs).

💡 Key Knowledge: Group 2 Solubility Trends

• Hydroxides [Mg(OH)₂ → Ba(OH)₂]: Solubility increases down the group. (Mg(OH)₂ is sparingly soluble; Ba(OH)₂ is soluble).
• Sulfates [MgSO₄ → BaSO₄]: Solubility decreases down the group. (MgSO₄ is soluble; BaSO₄ is insoluble).
• Even though Ba²⁺ ions are toxic, BaSO₄ can be ingested because its extreme insolubility prevents it from entering the bloodstream.

✅ Final Answers

% abundance ⁸⁷Sr = 7% (or 7.00%)
% abundance ⁸⁶Sr = 10% (or 10.00%)

❌ Common Errors

• Algebraic slip when expanding 86(17 − x) .
• Forgetting to divide by 100 or mixing fractional abundances with percentages.
• Swapping the final two answers around at the end.

❌ Common Traps in Back Titrations

• Missing the 1 : 2 ratio: Forgetting to divide moles of HCl by 2 when converting to Mg(OH)₂ (M4).
• Wrong subtraction: Adding the titration moles instead of subtracting them from initial moles to find the reacted quantity.
• Mᵣ Errors: Calculating Mᵣ of Mg(OH)₂ without doubling both oxygen and hydrogen atoms (must be 58.3).
• Sanity Check: Any final percentage over 100% automatically scores 0 for M6.

🧠 Top Tip: The Back Titration Map

Always draw a quick mental timeline:

[Total Acid Added] − [Excess Acid Reacted with NaOH] = [Acid Reacted with Tablet]

Once you have moles of acid consumed by the tablet, apply the chemical equation's stoichiometric ratio to get tablet active ingredient moles.