In Question 1 you measured the volume of carbon dioxide produced by a metal carbonate, MCO3, in order to identify M. In Question 2 you will identify another Group 2 metal, Q, by using a gravimetric method. When Group 2 carbonates are heated they decompose. QCO3(s) → QO(s) + CO2(g) FA 3 is the metal carbonate, QCO3.

1. (a)Method • Weigh the crucible with its lid and record the mass. • Add between 1.30g and 1.50 g of FA 3 into the crucible. Record the mass of crucible, lid and FA 3. • Place the crucible on the pipe-clay triangle on the tripod. Put the lid on the crucible and heat gently for approximately 1 minute. • Use tongs to remove the lid and heat the crucible strongly for approximately 5 minutes. Replace the lid and then leave to cool. • While the crucible is cooling, begin work on Question 3. • When cool, reweigh the crucible with its lid and contents. Record the mass. • Calculate and record the mass of FA 3 placed in the crucible. • Calculate and record the mass of residue left after heating. Keep the crucible and its contents for use in Question 3(b). Results I II III IV[4]
2. (b)(i)Calculate the number of moles of carbon dioxide produced during heating of FA 3. moles CO₂ = .............................................. mol [1][1]
3. (b)(ii)Use the mass of FA 3 in (a) and your answer to (b)(i) to calculate the relative atomic mass, Ar, of Q and hence identify Q. You should assume complete decomposition of QCO3. Ar of Q is .............................................. Q is .............................................. [4][4]
4. (c)Explain why the lid was placed on the crucible when the residue was left to cool. [1][1]
5. (d)(i)In order to decompose Group 2 carbonates, the solid must be heated strongly. In this experiment QCO3 was heated for a few minutes. Suggest an improvement to the method used that would ensure that decomposition was complete. [1][1]
6. (d)(ii)Suggest a chemical test to determine whether the decomposition of QCO3 was complete. State the expected observation if the decomposition was incomplete. Do not carry out this test. [1][1]
7. (e)(i)In your calculation in (b) you used the mass of QCO3 and assumed that it was all decomposed during the heating. Explain what effect incomplete decomposition would have on the calculated value of the Ar of Q. [1][1]
8. (e)(ii)A student suggested that you could use the mass of the residue, QO, rather than the mass of QCO3 in a calculation to identify Q. Explain why this method of calculating the Ar of Q is valid. [1][1]