Edexcel Jun 2010 Paper 6 Q3

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PMT 3 The purity of a sample of iron from a blast furnace may be determined by titration. The iron is contaminated with carbon and calcium silicate. A known mass of the impure iron is dissolved in dilute sulfuric acid to form a solution containing iron(II) ions. Portions of this solution are titrated with a solution of potassium manganate(VII) of known concentration. The steps of the experimental procedure are as follows. 1. Approximately 1.5g of a sample of the impure iron, in the form of a fine powder, was accurately weighed. The results are shown in Table 1. 2. The iron was transferred to a 250cm3 conical flask to which 50cm3 of dilute sulfuric acid (an excess) was added. The mixture was warmed to about 60C and then allowed to stand until the reaction was complete. 3. The mixture in step 2 was filtered and the residue was washed with a small volume of the dilute sulfuric acid. 4. All of the filtrate in step 3 was transferred to a 250cm3 volumetric flask. A further 50cm3 of the dilute sulfuric acid was added and then the volume was made up to the mark with distilled water. The resulting solution was mixed thoroughly. 5. 25.0 cm3 portions of the solution in step 4 were transferred to clean conical flasks and titrated with a potassium manganate(VII) solution of concentration 0.0220moldm3. The results are shown in Table 2. (a) Write an ionic equation for the reaction between iron and dilute sulfuric acid. State symbols are not required. (1) (b) How would you know when the reaction between the iron and the dilute sulfuric acid was complete? (1)*N36508A0616*<br /> PMT (c) The results of the experiment are given in the tables below. Table 1 Mass of weighing bottle + impure iron Mass of empty weighing bottle Mass of impure iron 11.22 g 9.74 g 1.48 g Solution in the burette: 0.0220moldm3 potassium manganate(VII) Solution in the flask: 25.00cm3 of solution containing iron(II) ions (step 5) Table 2 Titration number Burette reading (final) / cm3 Burette reading (initial) / cm3 Titre / cm3 Titres used ((cid:57)or (cid:56) ) Trial 30.00 23.10 24.80 24.45 23.20 6.65 0.05 2.10 1.45 0.25 23.35 (cid:56) (i) Complete Table 2 by filling in the missing data. Then mark with a tick ((cid:57)) those titres that will be used in the calculation of the mean titre and mark with a cross ((cid:56)) any titres that will be discarded. (ii) Calculate the mean titre in cm3. (2) (1) *N36508A0716* Turn over<br /> PMT (iii) The ionic half-equations for the reactions of the iron(II) ions and the manganate(VII) ions are given below. Fe2+Fe3+ + e MnO4+ 8H+ + 5eMn2+ + 4H2O Calculate the mass of iron in the original sample of impure iron and hence calculate the percentage by mass of iron in the sample. [The relative atomic mass of iron is 55.8] (4) *N36508A0816*<br /> PMT (d) Name the pieces of apparatus used to measure the 25.0cm3 solution containing iron(II) ions in step 5 and the 50cm3 sulfuric acid in steps 2 and 4. Explain why different apparatus is used in each case. (2) To measure 25.0cm3 of solutionTo measure 50cm3 sulfuric acidExplanation(e) Suggest why it was necessary to add such a large excess of sulfuric acid. (1)(f) State how you would detect the end-point of the titration. (1)(g) Explain why it is incorrect to use hydrochloric acid instead of sulfuric acid in this titration. (2)(Total for Question 3 = 15 marks) *N36508A0916* Turn over<br /> PMT BLANK PAGE 10 *N36508A01016*<br />

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