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.
When phosphorus(V) chloride, PCl5, and ammonium chloride are heated together, the compound P3N3Cl6 is
formed, together with HCl gas.
P3N3Cl6 has a cyclic structure, like the Kekulé structure of benzene.
i. Write an equation for the reaction of PCl5 and ammonium chloride to form P3N3Cl6.
[1]
ii. Calculate the percentage by mass of P in P3N3Cl6.
Give your answer to 2 decimal places.
percentage by mass of P = ................... % [2]
iii. Suggest one example of evidence that could show that P3N3Cl6 has a Kekulé structure rather than a
delocalised structure.
[1]
iv. In a molecule of P3N3Cl6 all the N and Cl atoms are bonded to P atoms.
Suggest a possible structure for a molecule of P3N3Cl6.
[2]
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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 61 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.
When phosphorus(V) chloride, PCl5, and ammonium chloride are heated together, the compound P3N3Cl6 is
formed, together with HCl gas.
P3N3Cl6 has a cyclic structure, like the Kekulé structure of benzene.
i. Write an equation for the reaction of PCl5 and ammonium chloride to form P3N3Cl6.
[1]
ii. Calculate the percentage by mass of P in P3N3Cl6.
Give your answer to 2 decimal places.
percentage by mass of P = ................... % [2]
iii. Suggest one example of evidence that could show that P3N3Cl6 has a Kekulé structure rather than a
delocalised structure.
[1]
iv. In a molecule of P3N3Cl6 all the N and Cl atoms are bonded to P atoms.
Suggest a possible structure for a molecule of P3N3Cl6.
[2]
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