A-LevelChemistryElectrochemistry9701/33 · May/June 2025 · Q114 Marks
Hydrogen peroxide, H2O2, reacts rapidly with acidified potassium manganate(VII), KMnO4. 5H2O2(aq) + 2MnO4¯(aq) + 6H+(aq) → 502(g) + 8H2O(l) + 2Mn2+(aq) You will determine the concentration of a solution of hydrogen peroxide. You will first dilute the solution and then carry out a titration with acidified potassium manganate(VII). FA 1 is aqueous hydrogen peroxide, H2O2. FA 2 is 0.0300 moldm¯³ potassium manganate(VII), KMnO4. FA 3 is 1.0 moldm¯³ sulfuric acid, H2SO4. (a) Method Dilution of FA 1 • Pipette 25.0 cm³ of FA 1 into the 250 cm³ volumetric flask. • Add distilled water to make 250 cm³ of solution. • Shake the flask thoroughly. • Label this diluted solution of hydrogen peroxide FA 4. Titration • Fill the burette with FA 2. • Rinse the 25.0 cm³ pipette with distilled water and then with FA 4. • Pipette 25.0 cm³ of FA 4 into a conical flask. • Use the 25 cm³ measuring cylinder to add 10cm³ of FA 3 to the conical flask. • Perform a rough titration and record your burette readings in the space below. The rough titre is = ______________ cm³. • Carry out as many accurate titrations as you think necessary to obtain consistent results. • Make sure your recorded results show the precision of your practical work. • Record, in a suitable form in the space for results, all your burette readings and the volume of FA 2 added in each accurate titration. Keep FA 1 for use in Questions 2 and 3. Keep FA 2 for use in Question 3.
- (a) ResultsResults I II III IV V VI VII[7]
- (b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm³ of FA 4 required ______________ cm³ of FA 2.[1]
- (c)(i)Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures.[1]
- (c)(ii)Calculate the amount, in mol, of manganate(VII) ions in the volume of FA 2 in (b). amount of MnO4¯ = ______________ mol [1][1]
- (c)(iii)Calculate the amount, in mol, of hydrogen peroxide in 25.0 cm³ of FA 4. amount of H2O2 = ______________ mol [1][1]
- (c)(iv)Calculate the concentration, in moldm¯³, of hydrogen peroxide in FA 1. concentration of H2O2 in FA 1 = ______________ moldm⁻³ [1][1]
- (d)A student suggests that the experiment would be more accurate if a 10cm³ pipette is used to measure FA 3 in place of the measuring cylinder. State whether the student's suggestion is correct. Explain your answer. Include a calculation as part of your explanation.[2]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [1] I Recording of results in the space for results; Unambiguous headings AND units for all recorded results: initial / FA 1 temperature / temp / T; final / maximum temperature / temp / T; temp(erature) change / rise / ΔΤ; Units: (°C) // °C with each heading OR °C with each piece of data.
- [1] II initial and final temperatures recorded for both experiments. AND clear indication of Experiments 1 and 2.
- [1] III All thermometer readings recorded to nearest .5 °C AND temperature changes correctly calculated.
- [1] IV Difference in temperature changes for Exps 1 and 2 are within the following ranges of the candidate's mean ΔΤ: Cand mean ΔΤ < 5.0, |ΔΤ1 – ΔΤ2| <0.5; Cand mean ΔΤ 5.25-10.0, |ΔΤ1 – ΔΤ2| <1.0; Cand mean ΔΤ 10.25-15.0, |ΔΤ1 – ΔΤ2| <1.5; Cand mean ΔΤ > 15.25, |ΔΤ1 – ΔΤ2| <2.0
- [3] V Difference between mean temperature change for candidate and supervisor rounded to the nearest 0.5 °C, δ: Sup mean ΔΤ < 5.0, δ <1.0; Sup mean ΔΤ 5.25-10.0, δ <1.5; Sup mean ΔΤ 10.25-15.0, δ <2.5; Sup mean ΔΤ 15.25-20.0, δ <3.5; Sup mean ΔΤ 20.25-25.0, δ <4.5
- [1] Correct calculation of the mean titre; Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm³.; Working/explanation must be shown OR ticks must be put next to the two (or more) accurate readings selected. The mean should be quoted to 2 d.p. and be rounded to nearest 0.01 cm³..
- [1] Significant figures; All answers in (c)(ii) – (c)(iv) are expressed to 3 or 4 sf.
- [1] Correctly calculates amount of MnO4¯ = ((b) / 1000) × 0.03 mol
- [1] Correctly uses amount of H2O2 = (c)(ii) × 2.5 mol
- [1] Correctly uses [H2O2] = (c)(iii) × 40 × 10 mol dm-3
- [1] M1 Student is not correct AND acid is in excess.
- [1] M2 Calculation to show n(H+) added = 0.02 mol AND calculation of n(H+) required = (c)(ii) × 3 OR = (c)(iii) x 1.2 AND n(H+) added more than / greater than / in excess of n(H+) required.
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