Titration can be used to determine the concentration of dissolved oxygen in samples of river water. The procedure for the experiment is given. step 1 Use five 50 cm³ graduated syringes, A, B, C, D and E, to collect five separate 30.0cm³ samples of river water. step 2 In the laboratory, carefully add 5.0 cm³ of 0.220 mol dm⁻³ manganese(II) sulfate, MnSO₄(aq), into syringe A and mix well. step 3 Add 5.0 cm³ of alkaline aqueous potassium iodide into syringe A and mix well. step 4 Add 10.0cm³ of dilute sulfuric acid into syringe A and mix well. step 5 Transfer the contents of syringe A into a 150cm³ conical flask. Rinse syringe A using 10 cm³ of distilled water and add washings to the conical flask. step 6 Carry out one accurate titration of all the contents in the conical flask with 0.00200 mol dm⁻³ aqueous sodium thiosulfate, Na₂S₂O₃(aq), using starch indicator. Repeat steps 2–6 for the samples in syringes B–E.

1. (a)Aqueous sodium thiosulfate can be prepared from Na₂S₂O₃•5H₂O(s).
2. (a)(i)Determine the mass, in g, of Na₂S₂O₃•5H₂O(s) required to prepare 500.0 cm³ of 0.00200 mol dm⁻³ Na₂S₂O₃(aq). mass of Na₂S₂O₃•5H₂O(s) = ...g [1][1]
3. (a)(ii)Identify the piece of apparatus that should be used to prepare 500.0 cm³ of 0.00200 mol dm⁻³Na₂S₂O₃(aq) after the required mass of Na₂S₂O₃•5H₂O(s) has been weighed out. [1][1]
4. (b)The graduations on each syringe are every 1.0 cm³.
5. (b)(i)Calculate the percentage error in the measurement of 5.0cm³ of alkaline aqueous potassium iodide by the syringe. Show your working. percentage error = [1][1]
6. (b)(ii)Place one tick (✔) in each row in Table 1.1 to show the effect, if any, of using a larger volume of alkaline aqueous potassium iodide. Table 1.1 | | greater effect | no effect | smaller effect | |:---|:---|:---|:---| | uncertainty of the measurement | | | | | percentage error of the measurement | | | | [1][1]
7. (c)The sample in the conical flask and the prepared solution of sodium thiosulfate are provided. Describe the following procedures for the experiment using syringe A.
8. (c)(i)Preparing the clean burette before taking any readings. [2][2]
9. (c)(ii)Carrying out the one accurate titration in step 6. [2][2]
10. (d)Suggest why the reaction mixture is mixed well in steps 2–4. [1][1]
11. (e)Draw a table for recording the titration results for the five samples in syringes A–E. [2][2]
12. (f)The overall reaction taking place in the experiment is shown. O₂(aq) + 4S₂O₃²⁻(aq) + 4H⁺(aq) → 2S₄O₆²⁻(aq) + 2H₂O(l) A student carries out the experiment and determines the mean titre to be 12.65 cm³. Calculate the concentration, in mol dm⁻³, of dissolved oxygen in the river water. concentration of dissolved oxygen = ... mol dm⁻³ [2][2]
13. (g)Freshly distilled water does not contain any dissolved oxygen. A student decides to run the procedure on a sample of freshly distilled water and at the end obtains a value of 2.26 × 10⁻⁵ mol dm⁻³ dissolved oxygen.
14. (g)(i)Suggest why the student did not get a value of 0 mol dm⁻³. Assume the procedure was carried out correctly. [1][1]
15. (g)(ii)Suggest how the value of 2.26 × 10⁻⁵ mol dm⁻³ could be used to improve the answer in (f). [1][1]
16. (h)Suggest why this method is unsuitable for samples of tap water that have been purified by chlorination and so contain Cl₂(aq). [1][1]