Chemistry 9701/52 — May/June 2019

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

(a)

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.

(i)

Name the independent variable.

Name the dependent variable.

(ii)

Apart from mass of ore, state a variable that should be controlled.

(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.

(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

(c)

Three experiments for each of the six ores are done and the final gas volumes are recorded in the table. All 18 experiments use exactly the same mass of ore.

metal	ore	Expt 1 volume of $\text{CO}_2$ / $\text{cm}^3$	Expt 2 volume of $\text{CO}_2$ / $\text{cm}^3$	Expt 3 volume of $\text{CO}_2$ / $\text{cm}^3$	calculated average volume of $\text{CO}_2$ / $\text{cm}^3$
barium	witherite	67	70	69	69
calcium	calcite	81	79	82	81
copper	malachite	47	45	46	46
iron	siderite	68	55	70	64
lead	cerussite	75	73	72	73
zinc	smithsonite	53	55	52	53

(i)

The student uses the results to obtain an average volume of $\text{CO}_2$ produced for each ore and records it in the table.

Suggest another reason why each ore is tested three times.

(ii)

The student made an error in processing the data to obtain the average volume of $\text{CO}_2$ produced for one of the ores.

State the error made by the student and calculate the correct value.

error

correct value = .............................. $\text{cm}^3$

(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.

(e)

Rhodochrosite is the carbonate ore of manganese and does not contain hydroxide ions.

(i)

$2.00\text{ g}$ of rhodochrosite produced $148\text{ cm}^3$ of carbon dioxide under room conditions in its reaction with excess hydrochloric acid.

\text{MnCO}_3 + 2\text{HCl} \rightarrow \text{MnCl}_2 + \text{H}_2\text{O} + \text{CO}_2

Calculate the percentage of $\text{MnCO}_3$ in rhodochrosite. Give your answer to three significant figures.

The molar volume of a gas is $24.0\text{ dm}^3$ under room conditions.
[ $A_r$ : Mn, 54.9; C, 12.0; O, 16.0]

percentage of $\text{MnCO}_3$ in rhodochrosite = .............................. %

(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.

(iii)

Explain how the students can ensure the results in (e)(ii) are as accurate as possible.

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