Edexcel Jan 2016 (IAL) Paper 6 Q3 (with explained solutions) |

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Edexcel Jan 2016 (IAL) Paper 6 Q3

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3 This question is about a student experiment to prepare crystals of iron(II) sulfate-7-water (FeSO4.7H2O) and then to determine the number of moles of water of crystallization in the sample which they have prepared. (a) Each student was given 5.00 g of iron filings which was added to excess dilute sulfuric acid, warmed and allowed to stand until no further reaction occurred. The resulting solution was cooled and filtered, and the required crystals were obtained from the filtrate. (i) Calculate the minimum volume of dilute sulfuric acid of concentration 2.00 mol dm3 required to react completely with 5.00 g of pure iron filings. The equation for this reaction is Fe(s) + H2SO4(aq) (cid:314) FeSO4(aq) + H2(g) D O N O T W R I T E I N T H I S A R E A (2) D O N O T W R I T E I (ii) Why was the reaction mixture filtered? (1)*P46942A0616* N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />
A E R A S I H T N I E T I R W T O N O D (iii) Describe how pure crystals of iron(II) sulfate-7-water are obtained from the filtrate. (2)(iv) One student obtained a yield of 89.5% from this preparation. Taking the formula of the crystals as FeSO4.7H2O, calculate the mass of iron(II) sulfate-7-water obtained by this student. Assume that the iron filings were pure. (3) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46942A0716* Turn over<br />
(b) A second student dissolved 6.75 g of their prepared crystals in about 150 cm3 of dilute sulfuric acid in a beaker and used this solution to prepare exactly 250.0 cm3 of a solution for titration. 25.0 cm3 samples of this final solution were further acidified with dilute sulfuric acid. These samples were titrated with potassium manganate(VII) solution to determine the number of moles of water of crystallization per mole of iron(II) sulfate. (i) Describe in outline how you would prepare the 250.0 cm3 of the solution for titration from the solution obtained by dissolving 6.75 g of the crystals in 150 cm3 of dilute sulfuric acid. (3) D O N O T W R I T E I N T H I S A R E A(ii) Suggest what would happen to the solution of iron(II) sulfate if it was prepared using distilled water, rather than dilute sulfuric acid as the solvent. Describe and explain what you would see. (2)(iii) Describe the end point of the titration. (1)*P46942A0816* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />
(iv) Using 6.75 g of their crystals and the method described in (b), the student obtained a mean titre of 25.35 cm3. The concentration of the potassium manganate(VII) solution was 0.0195 mol dm3 and the equation for the titration reaction is MnO4+ 8H+ + 5Fe2+ (cid:314) Mn2+ + 5Fe3+ + 4H2O Calculate the molar mass of the crystals and hence the number of moles of water of crystallization per mole of iron(II) sulfate in the students crystals. You must show your working. (4) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46942A0916* Turn over<br />
(c) A third student carried out the experiment described in (b) and found that there was 7.1 mol of water of crystallization per mole of the iron(II) sulfate. (i) The total experimental uncertainty associated with the determination of the molar mass is approximately0.9%. Use these data to show that the result obtained by this student is within this experimental uncertainty. (2)D O N O T W R I T E I N T H I S A R E A(ii) Most of the students in the class obtained values higher than the Data Book value of 7. Suggest a reason for this. (1)(Total for Question 3 = 21 marks) 10 *P46942A01016* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />
A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D BLANK PAGE A E R A S I H T N I E T I R W T O N O D *P46942A01116* 11 Turn over<br />

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Reject Mark Units incorrect or omitted Question Number 3(a)(i) Amount of iron = 5.00/55.8 (mol) (1) Acceptable Answer = amount H2SO4 (= 0.089606 / 8.9606 x 102 (mol)) Volume of 2.00 mol dm3 H2SO4 required = 5.00/55.82 = 0.044803 / 4.4803 x 102 dm3 OR = 44.803 cm3 (1) If Ar(Fe) = 56, volume = 0.044643 dm3 (= 44.643 cm3) TE on incorrect mol IGNORE all SF and rounding errors except 1 SF on final answer, and rounding errors affecting final answer Question Number 3(a)(ii) To remove solid impurities Acceptable Answer ALLOW Undissolved solids OR Insoluble impurities Reject To remove unreacted iron Mark Acceptable Answer Question Number 3(a)(iii) Evaporate the solution to crystallization point OR until crystals /solids begin to form ALLOW Concentrate the solution by evaporation OR Reduce the volume by 2575 % (1) (Cover solution and) allow to stand / cool and dry crystals between filter papers ALLOW Dry in a warm oven / desiccator (1) IGNORE Filtering Points relating just to recrystallization Reject Mark Just evaporate the solution OR Evaporate all water OR Distillation Dry the solution T>80oC Hot oven<br />
Question Number 3(a)(iv) Mr (FeSO4.7H2O) = 277.9 Acceptable Answer Reject Mark ALLOW = 278 (1) Maximum yield = 277.9 x 5.0055.8 (1) = 277.9 x 0.08961 (= 24.901 g) 89.5% yield = 0.895 x 277.9 x 5.00/55.8 = 22.287 (g) OR If Ar(Fe) = 56 and Ar(S) = 32 89.5% yield = 22.215 (g) (1) TE at each stage IGNORE SF except 1 SF Correct answer with no working scores 3 Additional Comment The 89.5% yield can be applied to the mass of iron, the mass of FeSO4.7H2O or the moles of FeSO4.7H2O to gain MP3<br />
Question Number 3(b)(i) These marks are standalone Acceptable Answer Transfer solution to a volumetric / graduated / standard flask (1) add washings (1) Make up to mark / line / 250 cm3 (with distilled water / dilute sulfuric acid) and then mix (1) ALLOW Different indication of mixing (e.g. swirl / invert / stir). Mixing must follow making up to mark IGNORE reference to weighing bottle and mixing when dissolving solid and washings from the weighing Reject Mark Question Number 3(b)(ii) Reject Mark red red Just oxidation Acceptable Answer (Pale green solution turns) yellow /orange / brown OR (Pale green solution forms) yellow /orange / brown (solution / cloudy solution / precipitate (1) Because (some of) the iron(II) / Fe2+ ions are oxidized (to iron(III) / Fe3+) OR iron(III) / Fe3+ ions are formed (from iron(II) / Fe2+ ions) ALLOW Fe(OH)3 formed (1)<br />
Question Number 3(b)(iii) (colourless /pale yellow to first permanent Acceptable Answer pale) pink Mark Reject (colourless /pale yellow to purple / mauve purple to pink pink / purple to colourless Reject Mark Question Number 3(b)(iv) Amount of MnO4 Acceptable Answer= 25.35 x 0.0195/1000 (1) = 4.94325 x 104 (mol) (ans*) Amount of Fe2+ in 250 cm3 = 5 x 10 x ans* (ans**) (1) = 0.024716 (mol) Mass of 1 mol of FeSO4.xH2O = 6.75ans** = 273.10 (g) (ans***) (1) IGNORE SF except 1 SF up to this point Correct molar mass with no working scores first 3 marks Moles of water of crystallization = (ans***151.9)/18 = 6.7333 = 6.7=7 (1) If Ar(Fe) = 56, x = 6.7278 TE at each stage Final answer = 7 with no working scores 0<br />
Reject Mark Question Number 3(c)(i) Mr (FeSO4.7H2O) = 277.9 Acceptable Answer uncertainty = 277.9 x 0.9/100 = ()2.501 ALLOW Mr (FeSO4.7H2O) = 273 (from 3b(iv) is used (uncertainty = ()2.458) OR Mr (FeSO4.7.1H2O) = 279.7 is used (uncertainty = ()2.517) (1) (cid:4666) answers = 277.92.5 are within experimental uncertainty) Using x = 7.1 gives Mr (FeSO4.7H2O) = 279.7 and this is within the uncertainty range (1) If neither mark is scored ALLOW Answer must be an integer so only needs to be in the range 6.6 to 7.4 for 1 mark IGNORE Calculations based on the percentage difference between 7.1 and 7 Question Number 3(c)(ii) The crystals were not dry Acceptable Answer OR Some of the iron(II) had been oxidized (to iron(III)) OR iron(II) / Fe2+ ions converted into iron(III) / Fe3+ ions IGNORE impurities / transfer errors / titration errors Reject Titration value too large Mark (Total for Question 3 = 21 marks)<br />

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