P is a solution containing Fe²⁺, NH₄⁺ and SO₄²⁻. NaOH → green precipitate insoluble in excess (Fe²⁺) + ammonia on heating, turning damp red litmus paper blue (NH₄⁺); dilute nitric acid → no observable change, then barium nitrate → white precipitate (SO₄²⁻).
Purple KMnO₄ (Q) oxidises Fe²⁺ to Fe³⁺:
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
That equation is where the 5 : 1 ratio used in (g)(i) comes from (the paper gives you the ratio). The titration is self-indicating — the first persistent pale pink is the end-point. Read the burette from the top of the meniscus. Sample readings (a school's own results): average titre 13.65 cm³ → 2.73×10⁻⁴ mol KMnO₄ → ×5 → 1.365×10⁻³ mol Fe²⁺ → 0.0546 mol/dm³.
Written observations are checked offline against the mark scheme's points using keyword/synonym patterns; the calculations are marked numerically (with tolerance). The feedback tells you what's missing so you can re-mark.