16 (B410U10-1) Examiner only 13. A sample of witherite, an ore of barium, contains barium carbonate, BaCO3. A geologist investigated the ore to determine the percentage of barium carbonate present by adding a sample of ore to excess acid and then finding how much acid had been neutralised. He followed these instructions. Stage 1 Add a known mass of ore to about 100 cm3 of 0.500 mol dm–3 hydrochloric acid in a beaker. Stir until no further reaction occurs. The geologist used 19.15 g of ore. Stage 2 Filter and then add more acid to make the total volume of the solution up to exactly 250 cm3. Stage 3 Titrate 25.0 cm3 samples of this solution against 0.100 mol dm–3 sodium hydroxide using a suitable indicator. The geologist used a mean volume of 27.80  cm3 of the sodium hydroxide to neutralise the acid. (a) Explain why the geologist filtered the mixture in Stage 2 of the process. [1] (b) State which piece of apparatus the geologist would use to make exactly 250 cm3 of solution in Stage 2. [1] (c) State why an indicator is used in Stage 3. [1] (d) State how many titrations the geologist should carry out in Stage 3. Give a reason for your choice. [1] © WJEC CBAC Ltd. (B410U10-1) 17 Examiner only (e) Write the equation for the reaction of barium carbonate with hydrochloric acid. [1] (f) Calculate the total number of moles of hydrochloric acid added to the sample of ore. [1] Number of moles added = ........................................................ mol (g) Calculate the number of moles of hydrochloric acid neutralised in each titration and hence the number of moles neutralised by the original sample of ore. [3] Number of moles neutralised by ore = ........................................................ mol QUESTION 13 CONTINUES ON PAGE 18. © WJEC CBAC Ltd. Turn over. 18 (B410U10-1) Examiner only (h) State the number of moles of barium carbonate present in the original sample of ore and hence calculate the percentage by mass of barium in the ore. [3] Percentage barium in the ore = ........................................................ % (i) The true value for the percentage of barium present in the ore is higher than that calculated in part (h). Suggest a possible reason for this. [1] END OF PAPER © WJEC CBAC Ltd. 13 Examiner only (B410U10-1) 19 © WJEC CBAC Ltd. Additional page.