12 (A410U10-1) Examiner only 13. A class of students is provided with a mixture of the strong base sodium hydroxide and the weak base sodium carbonate. They are asked to carry out an experiment to find the percentage by mass of each in the sample using the following method. • Prepare a standard solution of the solid mixture in a 250 cm3 volumetric flask. • Measure 25.00 cm3 of this mixture into a conical flask and add a small amount of an appropriate indicator. • Add 0.105 mol dm–3 hydrochloric acid from a burette whilst swirling the mixture until a permanent colour change occurs. At this point all the sodium hydroxide has reacted. • Record the results and calculate the volume required to reach the first end-point (volume A). • Add a few drops of a different indicator. • Add more of the hydrochloric acid from the burette whilst swirling the mixture until a permanent colour change occurs. At this point all the sodium carbonate has reacted. • Record the results and calculate the additional volume required to reach the second end- point (volume B). (a) The results of the titrations are shown below. © WJEC CBAC Ltd. 1 2 3 4 Initial burette reading / cm3 0.00 0.00 1.20 5.55 Burette reading at first end-point / cm3 22.35 22.00 23.25 27.55 Burette reading at second end-point / cm3 33.55 32.65 33.80 38.00 Volume required to reach first end-point (volume A) / cm3 22.35 Additional volume required to reach second end-point (volume B) / cm3 11.20 (A410U10-1) Turn over. 13 Examiner only © WJEC CBAC Ltd. (i) Complete the table and calculate the mean volume required to reach the first end- point, and the mean additional volume required to reach the second end-point. [3] Mean volume for first end-point = ...................................................................... cm3 Mean additional volume for second end-point = ...................................................................... cm3 (ii) The class teacher tells the students that the data show that the value for volume A is more reliable than the value for volume B. Give two reasons for this. [2] (iii) Calculate the mass of sodium carbonate present in the original solid mixture. [3] Na2CO3 + 2HCl 2NaCl + CO2 + H2O Mr 106.0 Mass = ....................................................... g 14 (A410U10-1) Examiner only © WJEC CBAC Ltd. (b) Explain how an indicator works and give reasons why two different indicators are used to identify the end-points for neutralisation of sodium hydroxide and sodium carbonate. Suggest how you would select appropriate indicators for this experiment. [6 QER] (A410U10-1) Turn over. 15 © WJEC CBAC Ltd. (c) One student suggests using 0.100 mol dm–3 ethanoic acid for this titration method, however his partner suggests that a different method would be needed. (i) Calculate the pH of 0.100 mol dm–3 ethanoic acid. [2] (Ka for ethanoic acid = 1.76 × 10−5 mol dm−3) pH = ....................................................... (ii) Suggest how the equivalence point for the titration of a weak acid and a weak base such as sodium carbonate can be found experimentally. [2] 18 Examiner only