A student is given 250.0 cm³ of solution containing a mixture of Fe2+ and Fe3+ ions. The student is asked to find the total mass of iron ions and the percentage by mass of Fe3+ in the solution by performing titrations with aqueous potassium manganate(VII), KMnO4. The student is told that the Fe3+(aq) ions can be reduced to Fe2+(aq) ions by reaction with zinc. The student is given the following instructions. : Calculate the mass of KMnO4 needed to make 500.0 cm³ of 0.0200 mol dm⁻³ KMnO4(aq). Record the mass of an empty plastic weighing boat (a small container used to hold solid samples). Add the calculated mass of KMnO4 to the weighing boat. Transfer the KMnO4 from the weighing boat into a 100 cm³ beaker. • Add 50 cm³ of distilled water to the beaker. • Transfer the mixture from the beaker into a 500.0 cm³ volumetric flask. • Make up to the graduation mark, dropwise, with distilled water.

1. (a)(i)Calculate the mass of KMnO4 needed to make 500.0cm³ of 0.0200 moldm⁻³ KMnO4(aq). [Α: Κ, 39.1; Mn, 54.9; O, 16.0] mass of KMnO4 needed = ...................................... g[1]
2. (a)(ii)The student used a balance accurate to two decimal places. Calculate the percentage error in weighing the mass of the KMnO4 by difference. If you were unable to calculate a value for 2(a)(i) use the mass 1.75g. This is not the correct answer to 2(a)(i). Show your working. percentage error = ...................................... %[1]
3. (a)(iii)The student noticed that some crystals of KMnO4 were stuck to the weighing boat after adding the KMnO4 solid to the beaker. State how the student should modify the instructions to ensure that the measured mass of KMnO4 was accurate.[1]
4. (a)(iv)Give two additional instructions that should be given to the student to ensure that the solution is prepared as accurately as possible. 1 2[2]
5. (b)(i)How should the burette be prepared for use before it is filled in step 1?[1]
6. (b)(ii)What must be done to ensure as accurate an end-point as possible?[1]
7. (c)(i)Identify an experimental weakness in step 2. Explain how this would affect the results.[1]
8. (c)(ii)How could this weakness be overcome?[1]
9. (d)(i)The results for each set of titrations are shown. set A rough titration 1 titration 2 titration 3 final volume/cm³ 18.40 17.25 34.55 18.00 initial volume/cm³ 0.65 0.15 17.25 0.95 titre/cm³ set B rough titration 1 titration 2 titration 3 final volume/cm³ 45.05 43.60 43.70 initial volume/cm³ 0.20 0.15 0.10 titre/cm³ Complete both tables and calculate an appropriate average titre for each set of results. The student could not carry out titration 3 in set B. Record the average titre to one decimal place. set A average titre = ................................ cm³ set B average titre = ................................ cm³[2]
10. (d)(ii)The reaction taking place during the titrations is shown by the equation. MnO4⁻(aq) + 5Fe2+(aq) + 8H+(aq) → Mn²+(aq) + 5Fe3+(aq) + 4H2O(l) Calculate the mass of Fe2+ ions in 100 cm³ of the reduced solution, produced in step 6, by using the appropriate average titre from (d)(i). Give your answer to three significant figures. [A: Fe, 55.8] mass of Fe2+ ions = ................................ g[2]
11. (d)(iii)Calculate the mass of Fe2+ ions in the original 250.0 cm³ Fe2+(aq)/Fe3+(aq) solution, using the appropriate average titre from 2(d)(i). mass of Fe2+ ions = ................................ g[1]
12. (d)(iv)Calculate the percentage by mass of Fe3+ ions in the original 250.0 cm³ Fe2+(aq)/Fe3+(aq) solution. percentage by mass of Fe3+ ions = ................................ %[1]
13. (d)(v)State what change could be made to the procedure to enable titration 3 to be carried out in set B.[1]