16   6 This question is about acids and bases. (a) State what is meant by a Brønsted‑Lowry base. (1) .................................................................................................................................................................................................................................................................................... (b) Write the ionic equation for the reaction between magnesium oxide and an acid. State symbols are required. (2) (c) Calculate the concentration of hydrogen ions, in mol dm –3, in a solution with a pH of 9.43 (1) (d) The pH of two salt solutions, J and K, are solution J pH = 5 solution K pH = 9 The solutions are equimolar. Which acids and bases could form the salts in solutions J and K? (1) A B C D Acid and base forming the salt in solution J Acid and base forming the salt in solution K HCl(aq) and NH3(aq) CH3COOH(aq) and NaOH(aq) HCl(aq) and NaOH(aq) CH3COOH(aq) and NH3(aq) CH3COOH(aq) and NaOH(aq) HCl(aq) and NaOH(aq) CH3COOH(aq) and NH3(aq) HCl(aq) and NH3(aq) Turn over 17   (e) The ionic product of water, Kw , varies with temperature as shown. Temperature /  °C Kw / mol2 dm–6 0 0.11 × 10–14 10 0.29 × 10–14 20 0.68 × 10–14 30 1.47 × 10–14 40 2.92 × 10–14 50 5.48 × 10–14 (i) Determine the value of Kw at 45 °C by plotting a suitable graph. You must show your working on the graph. (3) Kw at 45 °C = .............................................................. 18   (ii) The ionic product of water at 30 °C is 1.47 × 10–14 mol2 dm–6. Calculate the pH of water at this temperature. (3) (f) Hydrochloric acid, with a concentration of 0.100 mol dm–3, is added to 25.0 cm3 of 0.100 mol dm–3 aqueous sodium carbonate and the pH is measured. The titration curve is shown. 14 12 10 8 6 4 2 0 pH 0 Volume HCl(aq) / cm3 10 20 30 40 50 second equivalence point X first equivalence point The reaction takes place in two steps. The equation for the reaction taking place in the first step is Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq) Turn over 19   (i) Deduce a suitable indicator to identify the first equivalence point. Justify your answer using values from the Data Booklet. 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(ii) Write the equation for the reaction taking place at the second equivalence point. State symbols are not required. (1) (iii) Explain how the solution at point X on the graph can act as a buffer solution. 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(Total for Question 6 = 17 marks) 20   BLANK PAGE Turn over 21   7 This question is about chromium and some of its compounds. (a) The common oxidation numbers of chromium are +2, +3 and +6. Give a reason, in terms of ionisation energies, why chromium can show variable oxidation numbers. 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(b) The bonding in chromate(VI) ions, CrO4 2–, is similar to that in sulfate(VI) ions, SO4 2–. Draw a possible dot‑and‑cross diagram for a chromate(VI) ion. (2) 22   (c) A student added some pieces of zinc to an acidified solution of potassium dichromate(VI). Some standard electrode potentials are given in the table. Right‑hand electrode system E d / V Zn 2+(aq) + 2e–  Zn(s) –0.76 Cr 3+(aq) + e–  Cr 2+(aq) –0.41 Cr2O7 2–(aq) + 14H +(aq) + 6e–  2Cr 3+(aq) + 7H2O(l) +1.33 (i) Write the overall equation for the reduction of dichromate(VI) ions to chromium(III) ions by zinc in acid conditions. State symbols are not required. (2) (ii) Calculate E d cell for the reaction in (c)(i). (1) (iii) Predict whether or not a further reduction of chromium(III) ions to chromium(II) ions will occur. Justify your answer. 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Turn over 23   (iv) Aqueous solutions containing chromium(III) ions and chromium(II) ions have different colours. Explain why these solutions differ in colour. An explanation of the origin of the colours is not required. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 24   (d) An iron nail was analysed using the following outline procedure. • An iron nail was placed in a beaker and excess dilute sulfuric acid was added. • After all the iron had reacted to form iron(II) ions, the solution was made up to 1.00 dm 3 in a volumetric flask. • 25.0 cm3 portions of the solution were acidified and titrated with potassium dichromate(VI) solution of concentration 0.0167 mol dm –3. Results mass of iron nail = 3.54 g mean titre = 15.50 cm3 The table shows the percentage by mass of iron in four different brands of nail. Brand of nail Percentage by mass of iron A 92 B 94 C 96 D 98 Potassium dichromate(VI) in acid solution oxidises iron(II) ions as shown in the equation Cr2O7 2– + 14H + + 6Fe 2+ → 2Cr 3+ + 6Fe 3+ + 7H2O Determine, using the experimental data, the brand of nail that was analysed. (5) Turn over 25   (Total for Question 7 = 14 marks)