A-LevelChemistryAnalytical techniques9701/31 · May/June 2017 · Q214 Marks
Malachite is a basic form of copper carbonate in which copper hydroxide is also present. The accepted chemical formula of malachite is CuCO3.Cu(OH)2.H2O. When malachite is heated, it decomposes as shown. CuCO3.Cu(OH)2.H2O(s) → 2CuO(s) + CO2(g) + 2H2O(g) In this experiment, you will heat malachite to decompose it and use your results to obtain evidence about the accepted formula of malachite. FA 5 is malachite, CuCO3.Cu(OH)2.H2O.
- (a)Method Read through the method before starting any practical work. In the space below prepare a single table for your results of Experiments 1 and 2. Experiment 1 • Weigh a crucible with its lid and record the mass. • Add between 2.5g and 3.0g of FA 5 to the crucible. Weigh the crucible with FA 5 and lid and record the mass. • Place the crucible on the pipe-clay triangle. • Heat the crucible and contents gently for about two minutes, with the lid on. • Remove the lid and continue heating gently for about three minutes. • Replace the lid and leave the crucible and residue to cool for at least five minutes. Then reweigh the crucible and contents with the lid on. Record the mass. • While the crucible is cooling, you may wish to begin work on Question 3. • Calculate and record the mass of FA 5 used and the mass of residue obtained. • State the observation(s) you made while the reaction was taking place. Experiment 2 Repeat the method used in Experiment 1, using between 1.5g and 2.0 g of FA 5 in the second crucible. Results I II III IV V VI[6]
- (b)(i)Use your results from Experiment 1 to calculate the number of moles of copper oxide, CuO, obtained as residue. Use the Periodic Table on page 12 for any data you may require. moles of CuO obtained in Experiment 1 = ................................... mol[1]
- (b)(ii)Use your answer to (i), the equation on page 4 and the mass of FA 5 you used in Experiment 1, to calculate the relative formula mass, Mr, of malachite. Mr of malachite (from Experiment 1) = ...................................[1]
- (b)(iii)Use your results from Experiment 2 to calculate another value for the relative formula mass, Mr, of malachite. Mr of malachite (from Experiment 2) = ...................................[1]
- (b)(iv)Use data from the Periodic Table to calculate the relative formula mass, Mr, of malachite from its accepted formula, CuCO3.Cu(OH)2.H2O. Mr of malachite (from formula) = ...................................[1]
- (b)(v)If the relative formula mass of malachite obtained from either of your experiments is within 2.5% of the answer in (iv), this is good evidence that the accepted formula, CuCO3.Cu(OH)2.H2O, is correct. Show by calculation whether either of your experiments supports the accepted formula.[1]
- (c)(i)State one way of improving the accuracy of the experimental method, using the same masses of FA 5. Explain the benefit of your improvement.[3]
- (c)(ii)Explain why you would expect Experiment 1 to be more accurate than Experiment 2.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.
- [1] I: Table of data, to include: ∞ Unit “covering” all weighings, or given for each weighing ∞ No repeat headings (i.e. not two lists of weighings) ∞ Appropriate headings for the three weighings: Mass of crucible and lid Mass of crucible, lid and FA 5 (or “contents before heating”) Mass of crucible, lid and residue / CuO / contents after heating
- [1] II: Weighings recorded ∞ Six weighings recorded in the space provided. ∞ All weighings recorded to same number of decimal places (one or more) ∞ Label/heading to indicate which is Expt 1 and Expt 2
- [1] III: Both masses of FA 5 and residue, correctly subtracted ∞ Masses of FA 5 used recorded on page 4, correctly subtracted ∞ Masses of FA 5 used were between 2.5 – 3.0g and 1.5 - 2.0 g ∞ Masses of residue recorded on page 4, correctly subtracted
- [2] For assessment of accuracy, examiner must check and correct (if necessary) the masses of FA 5 used and of CuO obtained by the supervisor and by the candidate for Experiment 1. ∞ Examiner works out the ratio mass of FA5 / mass of CuO for the supervisor (2 dp) ∞ Examiner works out the ratio (mass FA 5: mass CuO) for the candidate (2 dp) ∞ Examiner calculates δ the difference between these two ratios. Award IV and V if δ < 0.08 Award IV if 0.08 < δ < 0.15
- [1] VI: Observations made during heating Solid goes black / black residue (formed) or reference to blue/green flame
- [1] α No of moles CuO = mass of residue / 79.5 ∞ Answer must be correct and expressed to 3 or 4 sig fig
- [1] α No of moles of FA 5 = answer (i) / 2 ∞ Mr = mass of FA 5 used / no of moles of FA 5
- [1] Mr = mass of FA5 used in Expt 2 / ( (mass of residue (CuO) in Expt 2 / 79.5) / 2 )
- [1] Mr of FA 5 calculated from Ar values = 239
- [1] Candidate should ∞ correctly calculate the 2.5% of Mr in (iv) = 5.98/6.0, and make a correct statement about the accuracy of the accepted formula, based on their result(s). or correctly calculate % difference for their result(s) from Mr in (iv) and correct comment
- [1] α heat (crucible and residue) to constant mass
- [1] ∞ heat more gently for longer period
- [1] ∞ cool in a desiccator
- [1] α to ensure that decomposition (of FA 5) is complete or to ensure that all the residue is CuO
- [1] ∞ to prevent escape of dust / smoke / solid (during heating)
- [1] Larger masses have lower percentage error in weighing
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