18   8 Reactions between calcium carbonate and acids produce carbon dioxide gas. CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O(l) This reaction can be used in an experiment to determine the molar volume of carbon dioxide. Procedure Step 1    Place 50 cm3 of 1 mol dm–3 nitric acid in a conical flask. Step 2    Set up the apparatus as shown in the diagram. Step 3    Place approximately 0.4 g of calcium carbonate in a test tube and weigh the test tube and its contents accurately. Step 4    Remove the bung from the conical flask. Step 5    Tip the calcium carbonate into the conical flask. Step 6    Replace the bung in the conical flask. Step 7    Once all the calcium carbonate has reacted, measure the volume of gas collected in the measuring cylinder. Step 8    Reweigh the test tube that had contained the calcium carbonate. Step 9    Repeat the experiment decreasing the mass of calcium carbonate added each time. delivery tube 100 cm3 measuring cylinder water bung nitric acid Turn over 19   (a) A major source of inaccuracy in this experiment occurs between Steps 5 and 6 when carbon dioxide gas is lost before the bung is replaced. (i) Identify a change to the acid in Step 1 that would reduce the volume of gas lost between Steps 5 and 6. Justify your answer. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Identify a change to the procedure in Steps 5 and 6 that would prevent loss of gas. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) A second source of inaccuracy results from the significant solubility in water of carbon dioxide. Give two improvements which together prevent this problem. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 20   (c) After appropriate improvements, the procedure was carried out and the results were plotted on a graph. A line of best fit was drawn. Volume of CO2 / cm3 0.00 0.10 0.20 0.30 0.40 100 90 80 70 60 50 40 30 20 10 0 Mass of CaCO3 / g 0.05 0.15 0.25 0.35 (i) Use the graph to find the molar volume of carbon dioxide, in dm3. You must show your working on the graph. [Assume molar mass of CaCO3 = 100 g mol–1] (3) Turn over 21   (ii) The procedure was carried out at a temperature of 22 °C and a pressure of 1.01 × 105 Pa. Calculate the theoretical molar volume, giving units with your answer. [The ideal gas equation is pV = nRT. Gas constant (R) = 8.31 J mol–1 K–1] (3) (Total for Question 8 = 12 marks)