Eduqas Chemistry for A Level Year 2: Student Bk

94 Eduqas A Level Chemistry Year 2: Component 1 5 The concentration of Fe 2+ (aq) can be determined by titrating against standard potassium dichromate(VI) solution. In one experiment, a 25.0 cm 3 sample of a solution containing Fe 2+ was titrated against 0.0250 mol dm –3 potassium dichromate(VI) solution in excess acid, when 12.5 cm 3 potassium dichromate(VI) solution was required to reach the end-point. Cr 2 O 7 2– (aq) + 14H + (aq) + 6Fe 2+ (aq) 2Cr 3+ (aq) + 7H 2 O(l) + 6Fe 3+ (aq) Calculate, to an appropriate number of significant figures, the concentration of the Fe 2+ (aq) solution. [3] 6 Sodium chlorate(I) is used in bleach. The concentration of sodium chlorate(I) in domestic bleach can be found by reacting the bleach with an acidified iodide solution to form iodine and then titrating with a thiosulfate solution. (a) Name a suitable indicator for this reaction. [1] (b) The equation for this reaction is given below. ClO – (aq) + 2H + (aq) + 2I – (aq) I 2 (aq) + Cl – (aq) + H 2 O(l) A 25.00 cm 3 sample of domestic bleach was diluted to 250.0 cm 3 in a volumetric flask. 25.00 cm 3 of this solution was added to an excess of acidified potassium iodide and the iodine produced reacted with 20.40 cm 3 of aqueous sodium thiosulfate of concentration 0·09200 mol dm –3 . (i) Write an equation for the reaction of I 2 with sodium thiosulfate solution. [1] (ii) Use the equation above, together with your equation in (b)(i), to calculate the concentration of sodium chlorate(I) in the original bleach sample to an appropriate number of significant figures. [4] 7 A sample of a copper-containing fungicide was analysed to determine its copper content. Firstly, it was reacted with excess potassium iodide: 2Cu 2+ + 4I – 2CuI + I 2 and the iodine produced was then titrated against sodium thiosulfate solution. I 2 + 2Na 2 S 2 O 3 2NaI + Na 2 S 4 O 6 (i) Name the indicator used for the titration and state the colour change at the end-point. [2] (ii) If a 31.2 g sample of the fungicide required 12.25cm 3 of sodium thiosulfate solution with concentration 0.100 mol dm –3 Na 2 S 2 O 3 to react with the liberated iodine, calculate the mass of copper in the sample and hence the %Cu by mass in the fungicide. [3] 8 The iron content of an alloy can be determined by a redox titration using acidified potassium dichromate(VI) solution, K 2 Cr 2 O 7 . A piece of alloy of mass 1.870 g was dissolved completely in acid to form Fe 2+ ions, and the solution made up to 250.0 cm 3 . A 25.00 cm 3 sample of this solution was titrated against acidified K 2 Cr 2 O 7 . This required 23.80 cm 3 of K 2 Cr 2 O 7 solution of concentration 0.0200 mol dm –3 for complete reaction. (i) The half-equations for the processes occurring are: Cr 2 O 7 2– + 14H + + 6e 2Cr 3+ + 7H 2 O Fe 3+ + e – Fe 2+ Write an ionic equation for the reaction between Fe 2+ ions and Cr 2 O 7 2– ions in acid solution. [1] (ii) Calculate the number of moles of Fe 2+ ions present in the 25.00 cm 3 sample used in the titration. [2] (iii) Calculate the percentage of iron in the original alloy sample. [2]