📖 What's the chemistry?
Q1 — Mohr's salt X
X is Mohr's salt, ammonium iron(II) sulfate,
(NH₄)₂Fe(SO₄)₂·6H₂O — a light green solid with three ions to find: NH₄⁺, Fe²⁺
and SO₄²⁻.
- Heating: the light green solid turns white as its water of crystallisation is driven off
(condensation near the mouth), then yellow on strong heating; the pungent gas (ammonia) turns damp
red litmus blue.
- Y = hydrogen peroxide oxidises Fe²⁺ → Fe³⁺ (solution turns red-brown) while it is itself
decomposed/reduced — effervescence of oxygen, which relights a glowing splint. A
redox reaction.
- NaOH on Fe²⁺ → dirty green precipitate, insoluble in excess (slowly darkens to red-brown in
air); warming with NaOH releases ammonia from NH₄⁺.
- NaOH after Y (Test 4) → red-brown Fe(OH)₃, confirming the oxidation.
- Acid + barium nitrate → white BaSO₄ precipitate → sulfate.
Q2 — iodometric titration
Cu²⁺ + iodide → white CuI + iodine (dark brown). The iodine is titrated with
thiosulfate P. As iodine is used up the colour fades; at light brown you add
the starch S (blue-black complex) so the last trace of iodine is visible — one more
drop and the dark colour vanishes, leaving the white CuI suspension.
- Why not add starch at the start? With so much iodine, the starch–iodine complex forms clumps that
release iodine too slowly — the end-point smears out.
- Chain: 22.30 cm³ × 0.120 → 0.00268 mol S₂O₃²⁻ = mol Cu²⁺ in 25 cm³ → ×40 → 0.107 mol → ×64 →
6.85 g → ÷9.50 → 72.1%.
Q3 — thermochemistry
The CaCO₃ is added at 3 min but the mixture is already losing heat, so the highest thermometer reading
underestimates the true rise. Extrapolating both straight lines back to 3 min gives
the corrected maximum (26.5 °C → rise 4.5 °C → 945 J → 27.0 kJ/mol).
Examiner tips
- Record burette readings to 0.05 cm³; average only concordant titres (never the rough).
- Always test and name the gas — “a gas is given off” scores nothing.
- Give calculated answers to 3 significant figures, with units.