Redox titrationsEdexcel A-Level Chemistry: Subtopic test

1. 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).

   (a)

   How many moles of manganate(VII) ions are in the mean titre?

   [1 mark]

   • A4.48 × 10⁻⁴ mol
   • B4.48 × 10⁻¹ mol
   • C2.24 × 10⁻³ mol
   • D4.48 × 10⁻⁷ mol

   (b)

   What is the concentration of the iron(II) sulfate solution?

   [1 mark]

   • A0.0179 mol dm⁻³
   • B0.448 mol dm⁻³
   • C0.00358 mol dm⁻³
   • D0.0896 mol dm⁻³

   (c)

   Explain why dilute sulfuric acid is used to acidify the iron(II) solution, rather than dilute hydrochloric acid.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What is the observation at the end-point of this titration?

   [1 mark]

   • AThe purple colour of the added solution disappears
   • BThe solution turns from orange to green
   • CThe solution turns from pale yellow to the first permanent pale pink
   • DThe solution turns from colourless to blue-black

   (b)

   How many moles of Fe²⁺ are in each 25.0 cm³ portion?

   [1 mark]

   • A1.12 × 10⁻⁴ mol
   • B5.60 × 10⁻⁴ mol
   • C5.60 × 10⁻³ mol
   • D5.60 × 10⁻⁷ mol

   (c)

   Calculate the mass of iron in the tablet.

   [2 marks]

   Total for question 2: 4 marks

3. 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).

   (a)

   Describe how the end-point of the titration is detected, and explain why the indicator is added when it is.

   [3 marks]

   (b)

   Calculate the concentration of sodium chlorate(I) in the original bleach, in mol dm⁻³.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student prepares a solution by dissolving 9.80 g of a sample of hydrated iron(II) ammonium sulfate, FeSO₄·(NH₄)₂SO₄·6H₂O (M = 392 g mol⁻¹), in dilute sulfuric acid and making it up to 250.0 cm³. 25.0 cm³ portions of this solution are titrated with 0.0200 mol dm⁻³ potassium manganate(VII) solution, and the mean titre is 24.80 cm³. The equation for the reaction is MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq).

   (a)

   Describe how the student should carry out the titration to obtain an accurate mean titre.

   [6 marks]

   (b)

   Calculate the percentage purity of the sample, and suggest one reason, other than impurities in the solid, why the value may be below 100%.

   [6 marks]

   Total for question 4: 12 marks

End of questions