A-LevelChemistryAtoms, molecules and stoichiometry9701/32 · May/June 2019 · Q210 Marks

When hydrated copper hydroxycarbonate, CuCO3.Cu(OH)2.yH2O is heated, it decomposes as shown. CuCO3.Cu(OH)2.yH2O(s) → 2CuO(s) + CO2(g) + (1 + y)H2O(g) In this experiment, you will heat hydrated copper hydroxycarbonate to decompose it. You will use your results to investigate the value of y. FB 4 is hydrated copper hydroxycarbonate, CuCO3.Cu(OH)2.yH2O.

(a)Method Weigh the crucible with its lid and record the mass. Add all the FB 4 from the container into the crucible. Weigh the crucible and lid with FB 4 and record the mass. Place the crucible and contents on the pipe-clay triangle. Heat the crucible and contents gently for approximately two minutes with the lid on. Use tongs to remove the lid and heat strongly for approximately three minutes. Replace the lid and leave the crucible and residue to cool for several minutes. While the crucible is cooling, you may wish to begin work on Question 3. When the crucible is cool, reweigh it with its lid and contents. Record the mass. Calculate and record the mass of FB 4 and the mass of residue obtained. Results I II III IV State the observations made while the decomposition of FB 4 was taking place.[4]
(b)(i)Calculate the number of moles of copper oxide, CuO, obtained as residue. moles of CuO obtained = .................................. mol[1]
(b)(ii)Use your results to calculate the relative formula mass, Mr, of hydrated copper hydroxycarbonate, CuCO3.Cu(OH)2.yH2O. Mr of CuCO3.Cu(OH)2.yH2O = ..................................[2]
(b)(iii)Use the Periodic Table to calculate the relative formula mass of CuCO3.Cu(OH)2. Mr of CuCO3.Cu(OH)2 = .................................. Use your answer to (ii) and this Mr to comment on the value of y in the formula of hydrated copper hydroxycarbonate.[2]
(c)State one way to improve the accuracy of the experiment, using the same mass of FB 4.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] 2(a) I: Table/list of data, to include values and correct headings and units:
[1] 2(a) Award II and III if δ < 0.05
[1] 2(a) Award III if 0.05 < δ < 0.10
[1] 2(a) IV: Observations made during heating
[1] 2(b)(i) Correctly calculated no. of moles of CuO
[1] 2(b)(ii) Correct use of mole ratio 1 : 2
[1] 2(b)(ii) Correctly uses n to calculate Mr of copper hydroxycarbonate,
[1] 2(b)(iii) Mr = 221
[1] 2(b)(iii) Appropriate comment on the value of y
[1] 2(c) Heat to constant mass (or description of procedure)

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