Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 3

✅ Correct Answer

Moles = 0.00165 mol (using Ar of Mg = 24.3) or 0.00167 mol (using Ar of Mg = 24).

📐 Calculation Steps

1. Identify mass of Mg = 0.04 g .
2. Use molar mass of Mg ( 24.3 g mol⁻¹ ).
3. Divide mass by molar mass: 0.04 ÷ 24.3 = 0.001646 mol .

❌ Common Errors

Dividing by the wrong molar mass value or making basic arithmetic slips. Note that examiners accept either 24 or 24.3 for the atomic mass of magnesium.

✅ Correct Answer

Moles of H₂ = 0.00165 mol (or whatever your answer was in part (a)(i)).

💡 Key Knowledge

Look at the stoichiometry of the balanced equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) . The molar ratio between Mg and H₂ is 1 : 1 .

🧠 Exam Technique (Error Carried Forward)

If your calculation in (a)(i) was incorrect, apply the 1:1 ratio to your incorrect value here to still pick up the available mark.

✅ Correct Answer

Expected volume = 40 cm³ (to 2 significant figures, or 40.2 cm³ ).

📐 Step-by-Step Calculation

1. Temperature conversion: 24 °C + 273 = 297 K .
2. Rearrange equation: V = nRT / p .
3. Substitute values: V = (0.001646 × 8.31 × 297) / 101000 = 4.022 × 10⁻⁵ m³ .
4. Convert units & round: Multiply by 10⁶ to convert m³ to cm³: 4.022 × 10⁻⁵ × 10⁶ = 40.2 cm³ (rounds to 40 cm³ ).

❌ Common Calculation Traps

• Forgetting to convert Celsius to Kelvin (+273).
• Failing to convert m³ into cm³ at the final step (forgetting the × 10⁶ factor).
• Using incorrect pressure units or failing to convert standard form correctly.

✅ Correct Answer

To identify anomalies and discard them, or to identify and use precise results.

💡 Key Knowledge

Replicating experiments increases reliability and allows anomalous results to be spotted and excluded from calculations. Never state "to improve accuracy" or "to make it more precise" as precision is limited by apparatus resolution.

✅ Correct Answer (Three Linked Pairs Required)

1. Issue: Hydrogen gas escapes before the bung is secured.
   Improvement: Use a sealed apparatus with a gas syringe or a conical flask with a sealed side-arm / gas syringe setup.
2. Issue: Magnesium ribbon is coated with an oxide layer.
   Improvement: Clean the magnesium ribbon with abrasive paper (emery paper) before weighing.
3. Issue: The mass of magnesium is too small (0.04 g), leading to a high percentage measurement uncertainty.
   Improvement: Use a more precise balance or a larger mass of magnesium.
4. Issue: The large 250 cm³ measuring cylinder has graduations too far apart to read accurately.
   Improvement: Use a smaller measuring cylinder, burette, or gas syringe.

🧠 Exam Technique & Guidance

You must provide a linked pair for each point: stating an issue alone or an improvement alone will not score full marks. You must give 3 distinct issues with their corresponding valid fixes to secure all 6 marks. Avoid vague statements like "impurities in general" or "human error".