2020 ACJC Prelim Paper 4 Answers

1 © ACJC 2020 9729/04/Prelim/2020 [Turn over ACJC Prelim 2020 Paper 4 Answers 1 Determination of percentage by mass of ethanedioic acid in a mixture of sodium ethanedioate and ethanedioic acid In this experiment, you are to determine the percentage by mass of ethanedioic acid in a mixture of sodium ethanedioate and ethanedioic acid. This experiment will be carried out in two parts. In part one, you will carry out a titration to find out the amount of ethanedioic acid, H2C2O4, present in the mixture. In part two, you will make use of the titration results to find out the total amount of ethanedioate ions, C2O42–, present in the mixture. You will then use the values found in these two parts to calculate the percentage by mass of ethanedioic acid in FA 1. FA 1 is a mixture of aqueous sodium ethanedioate, Na2C2O4, and ethanedioic acid, H2C2O4. FA 2 is 2.00 mol dm–3 NaOH FA 3 is 1.00 mol dm−3 sulfuric acid, H2SO4 FA 4 is 0.0200 mol dm−3 potassium manganate(VII), KMnO4. You are also provided with thymolphthalein solution. Part One: (a) Dilution of FA 2 solution As the NaOH solution provided is too concentrated, you will have to dilute it before conducting the experiment. 1. Fill the burette labelled FA 2 with FA 2. 2. Measure between 17.00 cm3 to 17.50 cm3 of FA 2 into a 250 cm3 volumetric (graduated) flask. 3. Record your burette readings in Table 1.1 below. 4. Fill the flask to the mark with deionised water, using a dropper when nearing the mark. 5. Stopper and mix the contents thoroughly by shaking to obtain a homogeneous solution. Label this solution FA 5. Table 1.1 final burette reading / cm3 17.00 initial burette reading / cm3 0.00 volume of FA 2 added / cm3 17.00 [1]

2 © ACJC 2020 9729/04/Prelim/2020 [Turn over (b) Titration of FA 1 with FA 5 1. Fill the burette labelled FA 5 with FA 5. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Add 3 to 4 drops of thymolphthalein solution. 4. Run FA 5 from the burette into the flask. The end-point is reached when the solution changes from colourless to a permanent pale blue colour. Record your titration results in Table 1.2 below. 5. Repeat steps 2 to 4 until consistent results are obtained and record your results in the table below. Table 1.2 final burette reading / cm3 25.00 41.00 initial burette reading / cm3 1.00 17.00 volume of FA 5 added / cm3 24.00 24.00 [4] From your titrations, obtain a suitable volume of FA 5, to be used in your calculations. Show clearly how you obtained this volume. Average titre = ½ (24.00 + 24.00) = 24.00 cm3 Suitable volume of FA 5 = ………24.00 cm3……….. [1] Part Two (c) Titration of FA 1 with FA 4 (There is n