This question is about the identification of a Group 2 carbonate.
A chemistry teacher found a bottle containing lumps of a white solid. The original label was missing from the bottle. However, someone had written ‘Group 2 carbonate’ on the bottle. The lumps of the anhydrous white solid were pure and dry.
The chemistry teacher tried to identify the carbonate with the help of three students.
The three students worked under identical conditions and shared the same weighing balance.
Student 1 recognised that if an acid is added to a carbonate, carbon dioxide is evolved. The student decided to measure the volume of carbon dioxide evolved when the Group 2 carbonate reacts with excess nitric acid.
The student knew that 1 mol of a Group 2 carbonate produces 1 mol of carbon dioxide.
Student 1 set up the apparatus shown below.
- Student 1 weighed out some of the Group 2 carbonate and added it to a 250 cm3 conical flask.
- Student 1 then added 100 cm3 of 0.200 mol dm−3 nitric acid to the conical flask and replaced the bung.
- Student 1 measured the volume of gas collected in the inverted measuring cylinder at room temperature and pressure (r.t.p.) when all the Group 2 carbonate had reacted.
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Student 1 obtained the results shown in Table 1.
Measurement | Value |
Mass of weighing bottle and carbonate / g | 13.247 |
Mass of empty weighing bottle / g | 12.431 |
Mass of carbonate used / g | ................... |
Volume of acid used / cm3 | 100 |
Volume of gas collected / cm3 | 225 |
Table 1
Complete Table 1 to show the mass of the carbonate used.
Calculate the amount, in moles, of carbon dioxide collected in the measuring cylinder at r.t.p.
Calculate the molar mass of the Group 2 carbonate to an appropriate number of significant figures and hence deduce the identity of the Group 2 metal.
Student 2 carried out the same experiment as Student 1, using the same mass of the Group 2 carbonate.
Student 2 made no errors in their measurements or calculations but obtained a value for the molar mass which was 10 g mol−1 greater than the value obtained by Student 1.
Give a reason why this mistake would not have affected Student 2’s result.
No calculation is required.
Explain how, if at all, this would affect the rate of reaction and the final volume of gas produced in the reaction.
Student 3 suggested a different experiment.
Student 3 realised that, by heating the carbonate, carbon dioxide would be lost and an oxide would remain.
Student 3 decided to measure the change in mass of the carbonate and to use this information to calculate its molar mass.
- Student 3 weighed an empty test tube.
- Using a spatula, Student 3 added some of the carbonate to the test tube.
- The test tube containing the carbonate was then weighed.
- The test tube and its contents were heated to constant mass.
- The results obtained by Student 3 are shown in Table 2.
Measurement | Value |
Mass of carbonate + test tube / g | 20.447 |
Mass of oxide + test tube / g | 20.205 |
Mass of empty test tube / g | 19.996 |
Table 2
Student 3 used the same balance as Student 1.
Give a reason why the mass of the carbonate measured by Student 3 has a greater percentage uncertainty than that measured by Student 1.
Student 3 noticed that on heating the test tube some solid was lost.
Explain how this would affect the calculated value for the molar mass of the Group 2 carbonate.
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