24   9 Pineapple juice contains the weak acids citric acid (C6H8O7) and ascorbic acid (C6H8O6). The amount of each compound in a sample of 150 cm3 of pineapple juice can be determined by titration. (a) Experiment 1 is designed to determine the total amount of acid. 10.0 cm3 samples of pineapple juice are transferred to separate conical flasks and titrated with a solution of sodium hydroxide of known concentration. The total amount of acid in the 150 cm3 sample of pineapple juice is 8.00 × 10–3 mol. (i) Give a reason why methyl orange would not be a suitable indicator to use in this titration. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) A student did not notice an air bubble in the tip of the burette before carrying out one of their accurate titrations. During this titration, the air bubble escaped. Explain the effect this mistake would have on the value of this titre. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 25   (b) Experiment 2 is carried out to determine the amount of ascorbic acid (C6H8O6) in the pineapple juice. An outline procedure for this experiment is given. Step 1 5.00 cm3 of the pineapple juice is added to a conical flask. Step 2 Deionised water, a small amount of HCl(aq), a few crystals of potassium iodide, KI, and 3 drops of starch solution are also added to the flask. Step 3 The contents of the flask are swirled to ensure the KI dissolves fully. Step 4 The resultant mixture is titrated with a solution of potassium iodate(V), KIO3(aq), of concentration 0.00100 mol dm–3. The reactions that take place are IO3 –(aq) + 5I–(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l) C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2H+(aq) + 2I–(aq) Only the ascorbic acid reacts with the iodine. (i) The end-point of the titration is when the starch changes colour. Explain how this occurs, including the colour change. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 26   (ii) The total amount of acid in the 150 cm3 sample is 8.00 × 10–3 mol. The mean titre in Experiment 2 using 5.00 cm3 of pineapple juice is 9.50 cm3. Calculate the mass of citric acid in the 150 cm3 sample. (5) Turn over 27   (c) While doing background research for the experiment, a student found that three other compounds, D, E and F, are often present in pineapple juice. Compound F Compound D CH3 H3C HO O O Compound E O OH HO O OH CH3 CH3 CH3 CH3 CH3 CH3 H3C CH3 H3C H3C Predict which one of these compounds is most likely to affect the result of Experiment 1 and hence predict the effect on the mass of citric acid calculated in (b)(ii). Justify your answer. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 9 = 14 marks)