A-LevelChemistryAtoms, molecules and stoichiometry9701/33 · Feb/Mar 2017 · Q212 Marks

You will carry out a second experiment to determine the concentration of hydrogen peroxide, FA 1, in moldm⁻³, by titration with acidified aqueous potassium manganate(VII). The equation for the reaction is given below. 2MnO₄⁻(aq) + 5H₂O₂(aq) + 6H⁺(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g) FA 1 is a solution of hydrogen peroxide, H₂O₂. FA 3 is 0.0300 mol dm⁻³ potassium manganate(VII), KMnO₄. FA 4 is dilute sulfuric acid.

(a)Method • Fill the burette with FA 3. • Pipette 25.0 cm³ of FA 1 into a conical flask. • Use the 25 cm³ measuring cylinder to add approximately 20 cm³ of FA 4 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 certain any recorded results show the precision of your practical work. • Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration. I II III IV V VI VII[7]
(b)From your accurate titration results, obtain a suitable value for the volume of FA 3 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm³ of FA 1 required cm³ of FA 3.[1]
(c)(i)Calculate the number of moles of manganate(VII) ions present in the volume of FA 3 calculated in (b). moles of MnO₄⁻ =[1]
(c)(ii)Calculate the number of moles of hydrogen peroxide present in 25.0 cm³ of FA 1. moles of H₂O₂ =[1]
(c)(iii)Using your answer to (ii) calculate the concentration, in mol dm⁻³, of hydrogen peroxide in FA 1. concentration of H₂O₂ in FA 1 = mol dm⁻³[2]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2017). The mark or mark type is shown in brackets.

[1] I initial and final burette readings and volume added recorded for rough titre AND accurate titre details tabulated
[1] II initial and final burette readings recorded and volume of FA 3 added recorded for each accurate titration • all headings and units correct for accurate titrations – initial/final (burette) reading/volume OR reading/volume at start/finish – titre OR volume FA 3 added/used – (cm³) OR / cm³ OR in cm³ by every entry
[1] III all accurate burette readings are recorded to the nearest 0.05 cm³
[1] IV final titre within 0.10 cm³ of any previous accurate titre
[3] V, VI and VII award V, VI and VII for δ ≤ 0.20 cm³ award V and VI for 0.20 cm³ < δ ≤ 0.30 cm³ award V for 0.30 cm³ < δ ≤ 0.50 cm³
[1] mean titre correctly calculated from clearly selected values: • candidate must average two (or more) titres where the total spread is < 0.20 cm³ • working must be shown or ticks must be put next to the two (or more) accurate readings selected • the mean should normally be quoted to 2 d.p. rounded to the nearest 0.01 Note: the candidate's mean will sometimes be marked as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy.
[1] M1 correctly calculates (0.030 x (b)) / 1000
[1] M2 correctly uses (i) × 5/2
[1] M3 correctly uses (ii) × 1000/25
[1] M4 all final answers to 3 or 4 sig. fig. (minimum two parts attempted)

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