1 This question is about compounds and ions of iron(II) and iron(III) that contain ethanedioate ions, C2O42–.
A student plans an investigation to find the number of waters of crystallisation, x, in a sample of hydrated iron(II)
ethanedioate, FeC2O4•xH2O.
The student decides to carry out a redox titration between solutions of iron(II) ethanedioate and potassium
manganate(VII) in acidic conditions.
i. In the titration, both iron(II) ions and ethanedioate, C2O42–, ions are oxidised.
Construct half-equations for the oxidation of iron(II) and ethanedioate ions.
Oxidation of iron(II) ions
Oxidation of ethanedioate ions
[2]
ii. The student prepares a 250.0 cm3 solution of iron(II) ethanedioate by dissolving 1.295 g of FeC2O4•xH2O, in
dilute sulfuric acid.
The student titrates 25.0 cm3 samples of this solution with 0.0200 mol dm–3 KMnO4 in the burette.
The student carries out a trial, followed by three further titrations.
The diagrams show the initial burette readings and the final burette readings for the student’s three further
titrations.
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, All burette readings are measured to the nearest 0.05 cm3.
Complete the titration table.
1 2 3
Final reading/ cm3
Initial reading/ cm3
Titre / cm3
[3]
iii. The uncertainty in each burette reading is ±0.05 cm3.
Calculate the percentage uncertainty for the titre in Titration 1.
percentage uncertainty = ................................................... %[1]
iv. In the titration, 5 mol of iron(II) ethanedioate reacts with 3 mol of manganate(VII) ions.
Analyse the student’s results to find the number of waters of crystallisation, x, in the hydrated iron(II)
ethanedioate, FeC2O4•xH2O.
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[6]
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, 2 This question is about the chemistry of compounds containing phosphorus.
Phosphine, PH3, is a poisonous gas.
i. Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.
Write the equation for this reaction.
[1]
ii. Aqueous silver nitrate, AgNO3, is reduced by PH3.
The unbalanced equation is shown below.
Balance the equation and use oxidation numbers to explain why this is a redox reaction.
........ AgNO3 + ........ PH3 + ........ H2O ........ Ag + ........ H3PO3 +........ HNO3
Explanation
[3]
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A student plans an investigation to find the number of waters of crystallisation, x, in a sample of hydrated iron(II)
ethanedioate, FeC2O4•xH2O.
The student decides to carry out a redox titration between solutions of iron(II) ethanedioate and potassium
manganate(VII) in acidic conditions.
i. In the titration, both iron(II) ions and ethanedioate, C2O42–, ions are oxidised.
Construct half-equations for the oxidation of iron(II) and ethanedioate ions.
Oxidation of iron(II) ions
Oxidation of ethanedioate ions
[2]
ii. The student prepares a 250.0 cm3 solution of iron(II) ethanedioate by dissolving 1.295 g of FeC2O4•xH2O, in
dilute sulfuric acid.
The student titrates 25.0 cm3 samples of this solution with 0.0200 mol dm–3 KMnO4 in the burette.
The student carries out a trial, followed by three further titrations.
The diagrams show the initial burette readings and the final burette readings for the student’s three further
titrations.
© OCR 2025. You may photocopy this page. 1 of 135 Created in ExamBuilder
, All burette readings are measured to the nearest 0.05 cm3.
Complete the titration table.
1 2 3
Final reading/ cm3
Initial reading/ cm3
Titre / cm3
[3]
iii. The uncertainty in each burette reading is ±0.05 cm3.
Calculate the percentage uncertainty for the titre in Titration 1.
percentage uncertainty = ................................................... %[1]
iv. In the titration, 5 mol of iron(II) ethanedioate reacts with 3 mol of manganate(VII) ions.
Analyse the student’s results to find the number of waters of crystallisation, x, in the hydrated iron(II)
ethanedioate, FeC2O4•xH2O.
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, Extra answer space if required.
[6]
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, 2 This question is about the chemistry of compounds containing phosphorus.
Phosphine, PH3, is a poisonous gas.
i. Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.
Write the equation for this reaction.
[1]
ii. Aqueous silver nitrate, AgNO3, is reduced by PH3.
The unbalanced equation is shown below.
Balance the equation and use oxidation numbers to explain why this is a redox reaction.
........ AgNO3 + ........ PH3 + ........ H2O ........ Ag + ........ H3PO3 +........ HNO3
Explanation
[3]
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