A student is provided with samples of six metal carbonate ores, known to be ores of the metals barium, calcium, copper, iron, lead and zinc. All the ores contain carbonate ions and some might also contain hydroxide ions. Each ore sample is ground up and a measured mass of the ore is reacted with an excess of dilute hydrochloric acid. The carbon dioxide produced by the reaction is collected over water. The volume of gas collected is recorded. Each ore is tested three times.

1. (a)(i)Name the independent variable. Name the dependent variable.[2]
2. (a)(ii)Apart from mass of ore, state a variable that should be controlled.[1]
3. (a)(iii)The student plans to collect the carbon dioxide produced over water. Draw a labelled diagram of the apparatus that could be used to do these experiments. The apparatus should allow the accurate recording of the carbon dioxide produced.[3]
4. (b)State one weakness in the procedure of collecting carbon dioxide over water. Suggest a change in apparatus that could be made to avoid this problem. weakness in procedure change in apparatus[2]
5. (c)(i)The student uses the results to obtain an average volume of CO2 produced for each ore and records it in the table. Suggest another reason why each ore is tested three times.[1]
6. (c)(ii)The student made an error in processing the data to obtain the average volume of CO2 produced for one of the ores. State the error made by the student and calculate the correct value. error correct value = cm³[1]
7. (d)A second student suggested that the percentage of carbonate ions could be obtained by titrating the ground-up ore samples with hydrochloric acid. Explain why a titration would not be a suitable method to determine the percentage of carbonate ions in some of these ore samples.[1]
8. (e)(i)Rhodochrosite is the carbonate ore of manganese and does not contain hydroxide ions. 2.00 g of rhodochrosite produced 148 cm³ of carbon dioxide under room conditions in its reaction with excess hydrochloric acid. MnCO3 + 2HCl → MnCl2 + H2O + CO2 Calculate the percentage of MnCO₃ in rhodochrosite. Give your answer to three significant figures. The molar volume of a gas is 24.0 dm³ under room conditions. [A₁: Mn, 54.9; C, 12.0; Ο, 16.0] percentage of MnCO3 in rhodochrosite = %[3]
9. (e)(ii)A teacher suggested that the mass change from the thermal decomposition of a ground-up sample of rhodochrosite could be used to determine the percentage of carbonate in the sample. The teacher told the students to strongly heat a ground-up sample of the ore in a crucible. State the measurements the students should make to determine the percentage of manganese carbonate in the sample of rhodochrosite.[1]
10. (e)(iii)Explain how the students can ensure the results in (e)(ii) are as accurate as possible.[1]

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