Redox titrationsEdexcel International A Level Chemistry: Subtopic test

1. 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)

   (a)

   What amount, in mol, of manganate(VII) ions is in the mean titre?

   [1 mark]

   • A4.48 × 10⁻⁴ mol
   • B4.48 × 10⁻¹ mol
   • C2.24 × 10⁻³ mol
   • D8.96 × 10⁻⁵ mol

   (b)

   What amount, in mol, of Fe²⁺ ions is in the 25.0 cm³ portion?

   [1 mark]

   • A8.96 × 10⁻⁵ mol
   • B4.48 × 10⁻⁴ mol
   • C1.12 × 10⁻² mol
   • D2.24 × 10⁻³ mol

   (c)

   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]

   Total for question 1: 4 marks

2. 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)

   (a)

   What amount, in mol, of thiosulfate ions is in the mean titre?

   [1 mark]

   • A1.86 mol
   • B1.86 × 10⁻⁴ mol
   • C1.86 × 10⁻³ mol
   • D9.30 × 10⁻⁴ mol

   (b)

   What is the concentration of the iodine solution?

   [1 mark]

   • A0.0744 mol dm⁻³
   • B0.0372 mol dm⁻³
   • C0.0186 mol dm⁻³
   • D0.372 mol dm⁻³

   (c)

   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]

   Total for question 2: 4 marks

3. 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.

   (a)

   Calculate the amount, in mol, of Fe²⁺ ions in the whole 250 cm³ of solution.

   [3 marks]

   (b)

   Calculate the relative formula mass of the crystals and the value of x in FeSO₄·xH₂O.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student determines x in FeSO₄·xH₂O by titrating 25.0 cm³ portions of a solution made from 5.00 g of the crystals in 250 cm³ against 0.0150 mol dm⁻³ potassium manganate(VII). The mean titre is 24.00 cm³ and the student calculates x = 7.0. The uncertainties of the apparatus are: burette ±0.05 cm³ for each reading (the titre needs two readings); 25.0 cm³ pipette ±0.06 cm³; balance ±0.005 g for each reading (the mass is found by difference using two readings).

   (a)

   Calculate the percentage uncertainty in each of the titre, the pipette volume and the mass of crystals, and the total percentage uncertainty in the experiment. State which measurement contributes the most.

   [6 marks]

   (b)

   Evaluate whether the student's result of x = 7.0 can be considered valid, and suggest improvements to the procedure.

   [6 marks]

   Total for question 4: 12 marks

End of questions