A-LevelChemistryAtoms, molecules and stoichiometry9701/21 · Oct/Nov 2018 · Q111 Marks

Iron pyrite, FeS2, has a yellow colour that makes it look like gold metal. The compound contains the ions Fe2+ and S22-.

(a)(i)Give the full electronic configuration of Fe2+.[1]
(a)(ii)Calculate the oxidation number of sulfur in the S22- ion. Assume that each sulfur atom in the ion has the same oxidation number. oxidation number of sulfur in the S2²- ion = ..........................[1]
(b)Describe the metallic bonding in gold.[2]
(c)(i)The sulfur dioxide produced during roasting would cause environmental consequences if released into the atmosphere. State and explain one of these environmental consequences.[2]
(c)(ii)Complete the equation to show the roasting of iron pyrite in oxygen. 4FeS2 + .......................... → 2Fe2O3 + ..........................[2]
(c)(iii)A sample of impure iron pyrite was roasted in oxygen. The composition of the mixture of solid products is shown. solid product | mass/g ---|--- Fe2O3 | 33.18 Au | 0.37 Calculate the mass of FeS2 present in the sample of impure iron pyrite. Assume that all the FeS2 was converted to Fe2O3 during the roasting process. (Mr: FeS2, 120.0; Fe2O3, 159.6) mass of FeS2 = .......................... g[2]
(c)(iv)Use your answer to (iii) to calculate the percentage by mass of gold in this sample of impure iron pyrite. Assume that gold is the only impurity in this sample of impure iron pyrite. Give your answer to two significant figures. (If you were unable to calculate an answer to (iii), use 55.00g as the mass of FeS2 in this calculation. This is not the correct answer.) percentage by mass of gold = .......................... %[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[1] 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ (4s⁰)
[1] –1
[2] M1 attraction/hold; M2 positive ions / cations AND delocalised electrons (may be seen in a labelled diagram)
[2] M1 acid rain; M2 • destroys / damages / weathers / erodes / buildings / statues OR • kills/harms fish / coral / plants / crops / trees / deforestation OR • leaches salts / ions (aluminium) from soil (into rivers / lakes) OR • leaches away soil nutrients OR • breathing difficulties OR • lowers pH/increases acidity of soil / rivers / oceans / seas
[2] balanced equation with 11O2 and 8SO2; M1: O2 and SO2; M2: 11 and 8
[2] M1 is for process of calculating number of moles of Fe2O3 (33.18 ÷ 159.6 (= 0.2079 mol)); M2 for correct use of stoichiometry and 120.0 with candidate's M1 (0.2079 × 4/2 × 120.0 = 49.89 (g))
[1] (0.37/(0.37+49.89)) = 0.74

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