Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 4

✅ Mark Scheme Answer

• Gas: Hydrogen / H₂ [1 mark]
• Test & Result: Burns with a (squeaky) pop (or putting out / extinguishing a lighted splint with a pop) [1 mark]

❌ Common Errors

• Writing just H instead of H₂ or "hydrogen" — strictly not allowed.
• Stating "glowing splint relights" (which is the test for oxygen, O₂).
• Saying "splint pops" without specifying a lighted/burning splint.

✅ Balanced Chemical Equation

Mg(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂(g)

• Correct formulae of reactants and products [1 mark]
• Correct balancing and state symbols [1 mark]

Alternative ionic equation accepted:
Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)

🧠 Exam Technique

• Always check the charge on the group 2 ion: Mg forms Mg²⁺, requiring two nitrate ions: Mg(NO₃)₂.
• State symbols were explicitly requested here: ensure you write (s) , (aq) , and (g) neatly.

✅ Balanced Chemical Equation

Mg + 4HNO₃ → Mg(NO₃)₂ + 2NO₂ + 2H₂O

• Correct species and balanced coefficients [1 mark]
• Note: State symbols are not required (and ignored even if incorrect).

💡 Key Knowledge: Redox Behaviour of Nitric Acid

Concentrated HNO₃ acts as a powerful oxidising agent rather than a typical acid reacting to give H₂:

• Magnesium is oxidised: Mg → Mg²⁺ + 2e⁻
• Nitrate is reduced: NO₃⁻ + 2H⁺ + e⁻ → NO₂ + H₂O
• Combining the two half-equations in a 1:2 ratio gives the stoichiometric equation directly.

✅ Required Observations

Gas mixture is brown before bubbling through the universal indicator solution, and colourless afterwards. [1 mark]

(Both observations required for the single mark)

💡 Why the Colour Changes

The decomposition equation is:
2Mg(NO₃)₂(s) → 2MgO(s) + 4NO₂(g) + O₂(g)

• NO₂ is a characteristic pungent brown gas.
• O₂ is a colourless gas.
• Before UI: mixture is brown due to NO₂. Once NO₂ dissolves in water, only colourless O₂ remains.

✅ Mark Scheme Points

• Gas colour change: The brown gas / NO₂ / nitrogen dioxide is removed / dissolves in the solution. [1 mark]
• Indicator colour change: Universal indicator turns red because nitrogen dioxide is an acidic gas (or forms nitric acid / HNO₃ / an acidic solution). [1 mark]

❌ Common Errors

• Failing to explicitly link the colour removal to the dissolution/removal of NO₂.
• Saying oxygen turns the indicator red. Oxygen is neutral!
• Forgetting that non-metal oxides (like NO₂) are acidic oxides.

✅ Correct Answer: B (236 cm³)

B: 236 cm³ is the only correct answer. [1 mark]

🧠 Exam Technique: Reading Inverted Cylinders

• The measuring cylinder is inverted. The top (closed end) is 0 cm³ and numbers increase downwards.
• Major marks are shown at 200 cm³ and 250 cm³. The distance between them is 50 cm³, split into 5 major divisions → each major division = 10 cm³.
• Each small graduation = 2 cm³.
• The bottom of the meniscus rests 3 small divisions below the 230 cm³ line:
  230 + (3 × 2) = 236 cm³ .
• Why distractors are wrong:
  • A (218 cm³): Assumes major marks are 5 cm³ and sub-divisions are 1 cm³.
  • C (257 cm³): Misreads scale upwards from 250 cm³ with wrong graduations.
  • D (264 cm³): Incorrectly reads upwards from the 250 cm³ graduation line.

❌ Calculation Traps & Common Pitfalls

• Missing the 2:1 ratio: Forgetting to divide moles of Mg(NO₃)₂ by 2. NO₂ was absorbed in the UI solution, so the gas collected is purely O₂!
• Incorrect Units: Writing units of volume ( cm³ or dm³ ) instead of molar volume ( cm³ mol⁻¹ or dm³ mol⁻¹ ) loses mark 4.
• Transfer of Error (TE): Full TE is allowed if you carried forward an incorrect volume from (b)(iii) (e.g. using 218, 257, or 264 cm³).

🧠 Sanity Check

Standard molar volume at standard temperature and pressure (STP / RTP) is typically around 22.4 to 24.0 dm³ mol⁻¹. Getting 23.6 dm³ mol⁻¹ fits this expected physical constant perfectly!

✅ Mark Scheme Answer

• The volume measurement is recorded to 3 significant figures (whereas all the other data are given to 4 significant figures). [1 mark]
  (Alternatively: the measuring cylinder volume can only be read to the nearest 1 cm³ / 236 cm³ has 3 s.f.)
• This is the least / lowest number of significant figures among the data used in the calculation. [1 mark]

🧠 Exam Technique: How to Answer "Why X s.f.?"

Always structure your answer in two clear steps:

1. Identify: Name the specific experimental measurement that has the lowest number of significant figures (here: volume = 236 cm³, 3 s.f., compared to mass = 2.966 g and Mᵣ = 148.3, both 4 s.f.).
2. Rule: State the golden rule: A calculated result can be no more precise than the least precise data value used (least number of s.f.).