A-LevelChemistryAnalytical techniques9701/51 · Oct/Nov 2023 · Q112 Marks

The concentration of dissolved oxygen in a sample of water can be measured using the following method. Manganese(II) hydroxide, Mn(OH)₂, is oxidised by the oxygen dissolved in a sample of water to form manganese(III) hydroxide, Mn(OH)₃. O₂(aq) + 4Mn(OH)₂(s) + 2H₂O(l) → 4Mn(OH)₃(s) The manganese(III) hydroxide then reacts with iodide ions to produce aqueous iodine. 2Mn(OH)₃(s) + 6HCl(aq) + 2KI(aq) → 2MnCl₂(aq) + 2KCl(aq) + I₂(aq) + 6H₂O(l) The amount of iodine produced is proportional to the amount of dissolved oxygen. 25.0cm³ of the solution containing aqueous iodine is transferred into a conical flask and titrated against 1.00 × 10⁻³moldm⁻³ sodium thiosulfate, Na₂S₂O₃. I₂(aq) + 2Na₂S₂O₃(aq) → Na₂S₄O₆(aq) + 2NaI(aq) The concentration of oxygen in water at different temperatures is shown in Table 1.2. The concentration value is missing for 25°C. Table 1.2 | temperature/°C | concentration of oxygen × 10⁻⁴/mol dm⁻³ | |---|---| | 0 | 4.58 | | 5 | 3.97 | | 10 | 3.20 | | 15 | 3.13 | | 20 | 2.82 | | 25 | | | 30 | 2.33 | | 35 | 2.15 | | 40 | 2.05 |

(a)(i)Complete Table 1.1 and determine the mean titre to be used in calculating the concentration of dissolved oxygen. Table 1.1 | | trial run | run 1 | run 2 | run 3 | |---|---|---|---|---| | final burette reading/cm³ | 27.30 | 28.10 | 28.25 | 26.95 | | initial burette reading/cm³ | 0.00 | 1.10 | 1.55 | 0.15 | | titre/cm³ | | | | | mean titre = .............................................. cm³ [2][2]
(a)(ii)Calculate the concentration of dissolved oxygen in the 25.0 cm³ of solution. Show your working. concentration of dissolved oxygen in 25.0 cm³ of solution = .............................................. moldm⁻³ [3][3]
(b)Suggest a suitable piece of apparatus for the transfer of 25.0cm³ of the solution containing aqueous iodine. [1][1]
(c)Water samples are collected in full sealed flasks. Explain why the sealed flask must be completely full. [1][1]
(d)(i)Plot a graph of concentration of oxygen (y-axis) against temperature (x-axis) on the grid. Use a cross (x) to plot each data point. Draw a smooth curve of best fit. [2][2]
(d)(ii)Use the graph to deduce the concentration of oxygen at 25°C. concentration of oxygen at 25 °C = .............................................. mol dm⁻³ [1][1]
(d)(iii)Circle the most anomalous point on the graph. Suggest an explanation for this anomaly. Assume that there was no error in measuring oxygen concentration. [2][2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[2] M1: 27.30, 27.00, 26.70, 26.80
M2: correct averaging of two (or more) titres within 0.10 cm³ of each other. (Expected answer = (26.70 + 26.80) / 2 = 26.75)
[3] M1: n(Na2S2O3(aq)) = (a(i) / 1000) × 0.001 (Expected answer = 26.75/1000 x 0.001 = 2.675 × 10⁻⁵)
M2: n(O2(aq)) = M1 / 4 (Expected answer = 2.675 × 10⁻⁵ / 4 = 6.6875 x 10⁻⁶)
M3: concentration of oxygen = M2 × 1000 / 25 = M2 x 40 (Expected answer = 6.6875 × 10⁻⁶ × 1000/25 = 2.675 × 10⁻⁴ mol dm⁻³)
Answer to at least 3SF
[1] (25.0 cm³) volumetric pipette
[1] to avoid any oxygen from (trapped) air (in the flask) reacting with / oxidising Mn(OH)₂
[2] M1: All points plotted correctly.
M2: smooth curve LOBF drawn.
[1] Correct value read from candidate's graph.
Value given to at least 3SF. (Expected value – 2.57 × 10⁻⁴ moldm⁻³)
[2] M1 Point most anomalous to the plotted line of best fit circled.
M2: For anomalous point below the LOBF The (actual) temperature > recorded/measured temperature (in the table). OR For anomalous point above the LOBF. The (actual) temperature < recorded/measured temperature (in the table).

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