Hydrated ethanedioic acid is a diprotic acid with the formula (COOH)2•xH2O where x is an integer. Ethanedioic acid reacts with manganate(VII) ions when heated. 5(COOH)2(aq) + 2MnO4¯(aq) + 6H+(aq) → 10CO2(g) + 2Mn2+(aq) + 8H2O(l) You will determine the value of x in (COOH)2•xH2O by titrating a solution containing ethanedioic acid with manganate(VII) ions. • FA 1 is 6.20 g dm⁻³ aqueous ethanedioic acid, (COOH)2•xH2O. • FA 2 is 0.0200 mol dm⁻³ potassium manganate(VII), KMnO4. • FA 3 is 1.0 mol dm⁻³ sulfuric acid, H2SO4·

1. (a)Method • Fill the burette with FA 2. • Pipette 25.0 cm³ of FA 1 into a conical flask. • Use the measuring cylinder to add approximately 20 cm³ of FA 3 to the conical flask. • Place the conical flask on a tripod and gauze and heat carefully until the temperature of the solution is approximately 70°C. • Remove the flame. • Carefully lift the hot conical flask and place it on the white tile under the burette. • Add FA 2 drop-wise for the first 2–3cm³. Any initial pink colouring may take several seconds to disappear. • If the reaction mixture turns brown, reheat it to about 70 °C. If the brown colour disappears, continue the titration. If the brown colour remains, discard the contents of the flask and begin a new titration. • The end-point is reached when a permanent pale pink colour is formed. Perform a rough titration with FA 2. Record your burette readings in the space below. The rough titre is ................................... cm³. • Carry out as many titrations as you think necessary to obtain consistent results. Make sure any recorded results show the precision of your practical work. Record all your burette readings and the volume of FA 2 added in each accurate titration.[7]
2. (b)From your accurate titration results, calculate a suitable mean value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm³ of FA 1 required ................................... cm³ of FA 2. [1][1]
3. (c)(i)Give your answers to (c)(ii), (c)(iii) and (c)(iv) to the appropriate number of significant figures. [1][1]
4. (c)(ii)Calculate the amount, in mol, of manganate(VII) ions, MnO4¯, in the volume of FA 2 calculated in (b). amount of MnO4¯ = ................................... mol [1][1]
5. (c)(iii)Calculate the amount, in mol, of ethanedioic acid that reacts with the manganate(VII) ions in (c)(ii). amount of (COOH)2 = ................................... mol Hence calculate the concentration, in mol dm⁻³, of ethanedioic acid in FA 1. concentration of (COOH)2 = ................................... mol dm⁻³ [2][2]
6. (c)(iv)Calculate the relative molecular mass, Mr, of the ethanedioic acid in FA 1. Mr = ................................... [1][1]
7. (c)(v)Calculate the value of x in (COOH)2•xH2O. Show your working. x = ................................... [1][1]
8. (d)Explain why it is necessary to add FA 3 in each titration. [1][1]