WJEC A-Level Chemistry Unit 4, June 2025: Question 1

✅ Correct Answer

(Concentrated) nitric acid and (concentrated) sulfuric acid

💡 Key Knowledge

• Electrophile generated: the nitronium ion ( NO₂⁺ ).
• Role of H₂SO₄ : Acts as a Brønsted-Lowry acid catalyst, protonating HNO₃ to generate the active electrophile:
  HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻

❌ Common Errors

• Omitting sulfuric acid entirely and naming only nitric acid.
• Stating "dilute" acids; dilute acids will not generate the electrophile in sufficient concentration.
• Writing "nitrous acid" ( HNO₂ ) instead of nitric acid ( HNO₃ ).

🧠 Exam Technique

Always state both the specific reagents and their concentrations (e.g., concentrated HNO₃ and concentrated H₂SO₄). Whenever an exam asks for "reagent(s)" in nitration, examiners expect both components of the nitrating mixture.

✅ Correct Answer

Award [1 mark] for either of the following:

• The melting temperature will be lower (than 80 °C).
• It will melt over a range of temperatures (rather than a sharp melting point).

💡 Key Knowledge

• A completely pure organic solid has a sharp, well-defined melting temperature.
• Soluble impurities (such as moisture/water in a damp sample) disrupt the regular crystal lattice, weakening intermolecular forces.
• This lattice disruption causes the solid to melt below the literature value and over a broad temperature range.

❌ Common Errors

• Thinking water will "cool" the sample or raise the melting point due to high boiling point of water.
• Stating merely that the melting temperature "changes" without qualifying that it decreases or broadens.

🧠 Exam Technique

Use the standard organic chemistry phrasing: "lowers the melting point and broadens the melting range." Either description scores the single mark, but stating both guarantees you hit the mark scheme terminology.

Step 1: Calculate moles of reactant (TNT)
Mass of TNT = 1.00 g, Relative molecular mass (Mᵣ) = 227
Moles of TNT = mass / Mᵣ = 1.00 / 227 = 0.004405... mol (or 4.41 × 10⁻³ mol)

Step 2: Determine total moles of GASEOUS products per mole of TNT
Reaction equation:
TNT → ³/₂ N₂(g) + ⁵/₂ H₂O(g) + ⁷/₂ CO(g) + ⁷/₂ C(s)
Count only products with state symbol (g):
Total gas moles = ³/₂ + ⁵/₂ + ⁷/₂ = 15/2 = 7.5 mol of gas per 1 mole of TNT.
Note: Solid carbon, C(s), must NOT be included!

Step 3: Calculate total moles of gaseous products
Moles of gas = 7.5 × 0.004405... = 0.03304 mol (or 0.0331 mol to 3 s.f.)

✅ Summary of Final Answer

Number of moles = 0.0331 mol (or 3.31 × 10⁻² mol)

Acceptable range: 0.0330 to 0.0331 mol based on intermediate rounding.

❌ Common Calculation Traps

• Including solid carbon: Summing all stoichiometric coefficients gives (3 + 5 + 7 + 7)/2 = 11 moles, yielding 0.0485 mol. This loses the second mark because C is a solid, not a gas.
• Premature rounding: Rounding 0.004405 to 0.004 too early can lead to 0.03 mol, which loses marks for precision.
• Forgetting fractional coefficients: Missing the "/2" in the coefficients.