Redox titrationsEdexcel A-Level Chemistry: Mind map

Exam questions on Redox titrations

1. A student titrates 25.0 cm³ portions of acidified iron(II) sulfate solution with 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) + 4H₂O(l) + 5Fe³⁺(aq).
   Explain why dilute sulfuric acid is used to acidify the iron(II) solution, rather than dilute hydrochloric acid.2 marks
2. A student determines the mass of iron in an iron supplement tablet. One tablet is dissolved in dilute sulfuric acid and the solution is made up to 250.0 cm³. 25.0 cm³ portions are titrated with 0.0100 mol dm⁻³ potassium manganate(VII) solution and the mean titre is 11.20 cm³. The equation for the reaction is MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq). The relative atomic mass of iron is 55.8.
   Calculate the mass of iron in the tablet.2 marks
3. A technician determines the concentration of sodium chlorate(I), NaOCl, in a bleach. 10.0 cm³ of the bleach is diluted to 250.0 cm³ in a volumetric flask. A 25.0 cm³ portion is added to excess potassium iodide solution and acidified, and the iodine liberated is titrated with 0.100 mol dm⁻³ sodium thiosulfate solution. The mean titre is 19.40 cm³. The equations are OCl⁻(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + Cl⁻(aq) + H₂O(l) and I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq).
   Describe how the end-point of the titration is detected, and explain why the indicator is added when it is.3 marks