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.
© OCR 2025. You may photocopy this page. 1 of 113 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.
© OCR 2025. You may photocopy this page. 2 of 113 Created in ExamBuilder
, Extra answer space if required.
[6]
© OCR 2025. You may photocopy this page. 3 of 113 Created in ExamBuilder
, 2 Hydrogen gas is manufactured by the chemical industry from the reversible reaction of methane and steam,
shown below.
CH4(g) + H2O(g) ⇌ 3H2(g) + CO(g) ΔH = +195 kJ mol–1
Average bond enthalpies are shown in the table.
Bond H–H O–H C=O
–1
Average bond enthalpy / kJ mol +436 +464 +1077
i. Why do all average bond enthalpies have a positive value?
[1]
ii. Determine the C–H bond enthalpy, in kJ mol–1, using the information above.
C–H bond enthalpy = ................................................... kJ mol–1[3]
iii. Hydrogen gas is being considered as a household fuel to replace methane.
The enthalpy change of formation, ΔfH, for H2O(l) is –285.8 kJ mol–1.
Determine the energy released when 60.0 m3 of hydrogen is used as a household fuel at RTP.
Give your answer to 3 significant figures and in standard form.
energy released = ................................................... kJ [2]
© OCR 2025. You may photocopy this page. 4 of 113 Created in ExamBuilder
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 113 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.
© OCR 2025. You may photocopy this page. 2 of 113 Created in ExamBuilder
, Extra answer space if required.
[6]
© OCR 2025. You may photocopy this page. 3 of 113 Created in ExamBuilder
, 2 Hydrogen gas is manufactured by the chemical industry from the reversible reaction of methane and steam,
shown below.
CH4(g) + H2O(g) ⇌ 3H2(g) + CO(g) ΔH = +195 kJ mol–1
Average bond enthalpies are shown in the table.
Bond H–H O–H C=O
–1
Average bond enthalpy / kJ mol +436 +464 +1077
i. Why do all average bond enthalpies have a positive value?
[1]
ii. Determine the C–H bond enthalpy, in kJ mol–1, using the information above.
C–H bond enthalpy = ................................................... kJ mol–1[3]
iii. Hydrogen gas is being considered as a household fuel to replace methane.
The enthalpy change of formation, ΔfH, for H2O(l) is –285.8 kJ mol–1.
Determine the energy released when 60.0 m3 of hydrogen is used as a household fuel at RTP.
Give your answer to 3 significant figures and in standard form.
energy released = ................................................... kJ [2]
© OCR 2025. You may photocopy this page. 4 of 113 Created in ExamBuilder
