Redox titrationsEdexcel International A Level Chemistry: Flashcards

Exam questions on Redox titrations

1. 25.0 cm³ portions of an iron(II) sulfate solution are acidified with dilute sulfuric acid and titrated against 0.0200 mol dm⁻³ potassium manganate(VII) solution. The mean titre is 22.40 cm³. The equation for the reaction is: MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l)
   Explain why dilute sulfuric acid, rather than hydrochloric acid, is used to acidify the iron(II) solution, and state the colour change at the end-point.2 marks
2. 25.0 cm³ portions of an iodine solution are titrated against 0.100 mol dm⁻³ sodium thiosulfate solution, adding starch indicator near the end-point. The mean titre is 18.60 cm³. The equation for the reaction is: I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)
   State when the starch indicator is added in this titration, explain why it is added then, and state the colour change at the end-point.2 marks
3. 5.00 g of hydrated iron(II) sulfate crystals, FeSO₄·xH₂O, are dissolved in dilute sulfuric acid and made up to 250 cm³ in a volumetric flask. 25.0 cm³ portions of this solution are titrated against 0.0150 mol dm⁻³ potassium manganate(VII) solution and the mean titre is 24.00 cm³. The equation is: MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l). Relative atomic masses: H = 1.0, O = 16.0, S = 32.1, Fe = 55.8.
   Calculate the amount, in mol, of Fe²⁺ ions in the whole 250 cm³ of solution.3 marks