A-LevelChemistryAtoms, molecules and stoichiometry9701/51 · Oct/Nov 2020 · Q113 Marks

Aqueous potassium manganate(VII) can be used to determine the amount of iron present in a sample of iron wire by redox titration. Before potassium manganate(VII) can be used, its concentration must be determined using aqueous sodium ethanedioate made from the hydrated solid Na2C2O4.2H2O.

(a)(i)Calculate the mass of Na2C2O4•2H₂O required to make 250.0 cm³ of 0.200mol dm-³ sodium ethanedioate standard solution. [A: Na, 23.0; C, 12.0; O, 16.0; Η, 1.0][1]
(a)(ii)Describe how the student should accurately prepare 250.0 cm³ of 0.200 mol dm-³ sodium ethanedioate standard solution from the weighed sample of Na2C2O4•2H2O of mass calculated in (a)(i). In your description you should include the names and capacities of any apparatus used.[2]
(b)Ethanedioate ions, C2O42-(aq), react with manganate(VII) ions, MnO4¯(aq), according to the ionic equation shown. 2MnO4¯(aq) + 16H+(aq) + 5C2O42-(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) 25.0 cm³ of 0.200 mol dm¯³ C2O42-(aq) required 18.40 cm³ MnO4¯(aq) for complete reaction. Calculate the concentration of the aqueous potassium manganate(VII). Give your answer to three significant figures.[3]
(c)(i)Circle the titres the student should use to obtain the most accurate value for the volume of 0.0200 mol dm³ KMnO4 that is needed to react with 25.0cm³ of the prepared iron solution. Explain your answer.[1]
(c)(ii)The burette used for the titration has graduations every 0.10 cm³. Calculate the maximum percentage error in the titre of titration 2. Show your working.[1]
(c)(iii)Derive an expression to show how you would calculate the percentage by mass of iron in the iron wire. Use x to represent the average titre and y to represent the mass of iron wire used.[2]
(c)(iv)The student left the solution of Fe2+(aq) in sulfuric acid without a stopper for a few days. The student repeated the titration and found that the average titre was lower. Suggest why.[1]
(c)(v)In step 2, the sulfuric acid is used to dissolve the iron in the iron wire. Suggest the other function of the sulfuric acid in this experiment.[1]
(d)Name an appropriate piece of apparatus to measure the volume of sulfuric acid in step 2. Give a reason for your answer.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] 1(a)(i) = 0.200 × 250 / 1000 × 170.0 = 8.5 g
[2] 1(a)(ii) M1: dissolve a known mass / mass in (a)(i)) / solid in (distilled water), less than 250 cm³ (if stated), (in a suitable container)
M2: transfer/add the solution to a 250 cm³ volumetric / graduated flask (with washings) AND make up to the mark with (distilled) water.
[3] 1(b) M1: number of moles C2O42-(aq) = 25 / 1000 × 0.200 = 0.005 moles
M2: number of moles MnO4¯(aq) = answer to (b)(i) × 2 / 5 = 0.002 moles
M3: concentration = 0.002 moles × 1000/18.4 = 0.109 mol dm-3
[1] 1(c)(i) titres 2 and 4 AND they are concordant/ within 0.1 cm³
[1] 1(c)(ii) (2 × 0.05)/44.30 × 100 = 0.226%
[2] 1(c)(iii) M1: moles Fe2+ in 250 cm³ = (0.02 x/ 1000) × 5 × 10 or x (1 × 10-3)
M2: percentage is 5.58 x/y calculated correctly
[1] 1(c)(iv) fewer moles of Fe2+ present in the solution (as some would have oxidised to Fe3+ which would not react with the KMnO4)
[1] 1(c)(v) to provide H+ ions / protons for the titration OR To prevent (hydrolysis of) Fe2+ producing a precipitate
[1] 1(d) measuring cylinder, as the acid is in excess / accuracy of the measurement is not important