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CIE Jun 2014 v2 Paper 3 Q2
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![As well as hydrogen carbonate ions, water that has passed through rocks also contains a range of metal ions such as Ca2+. The presence of Ca2+ ions leads to the water forming scum with certain types of soap. Soluble metal carbonates can be added to remove these ions. One such soluble carbonate is M2CO3.10H2O. PMT In this experiment you will heat a sample of the carbonate salt to remove the water of crystallisation and leave the anhydrous salt, M2CO3. By working out how much water was lost you will be able to calculate the relative formula mass of M2CO3.10H2O and hence identify M. FB 3 is the hydrated salt, M2CO3.10H2O. (a) MethodWeigh a clean, dry crucible and its lid. Transfer all the FB 3 into the crucible and put the crucible lid on. Reweigh the crucible, the lid and the contents. Place the crucible on the pipe-clay triangle. Heat the crucible very gently for 5 minutes. Remove the lid and heat strongly for a further 5 minutes. Allow the crucible to cool. You should start Question 3 while cooling is taking place. When the crucible is cool enough to handle, reweigh the crucible, the lid and the contents. Record, in an appropriate form in the space below, all your weighings and calculations including the mass of FB 3 used and the mass of water lost. II III IV [4] (b) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Use the mass of water lost on heating to calculate the number of moles of hydrated salt, M2CO3.10H2O, in the initial sample. [Ar: H, 1.0; O, 16.0]UCLES 2014 9701/32/M/J/14 moles of M2CO3.10H2O =mol<br />
(ii) Calculate the relative formula mass of M2CO3.10H2O, FB 3. PMT relative formula mass of M2CO3.10H2O =(iii) Identify M in the salt, FB 3. [Ar: H, 1.0; Li, 6.9; C, 12.0; O, 16.0; Na, 23.0; K, 39.1; Rb, 85.5; Cs, 133] II III IV M is. [5] (c) (i) State the maximum error in the mass of FB 3 you recorded in (a). (ii) Calculate the maximum percentage error in the mass of water lost in (a). maximum error in the mass of FB 3 =g percentage error = % [1] (d) Suggest a change to the method that would improve the accuracy of this experiment. Explain your answer.[2]UCLES 2014 9701/32/M/J/14 [Total: 12] II III [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_v2_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 4 Mark Scheme GCE AS/A LEVELMay/June 2014 Syllabus 9701 Paper 32 Question Sections Indicative material Mark Total (a) PDO Display I Shows clearly mass of empty crucible, mass of crucible + FB 3, mass of crucible and residue with correct units. (mass/g or (g) or in g) Recording MMO Quality II Records all weighings consistently to at least 1 dp and records the correctly calculated mass of FB 3 used and water lost. Units must be correct and headings must be unambiguous. Calculate mass of water lost/mass of FB 3 to 2 dp and compare with Supervisor value. Award III and IV if0.05 Award IV only if 0.05 <0.08 (b) (i) ACE Interpretation mass water lost 18 (expression) or correct moles of water (ii) Final answer must be to 2 to 4 sf II moles of water divided by 10 or correct answer III Mr in (ii) from mass/ans (i) IV Calculates Ar of M Ar = M 240 (iii) Conclusion V Choice of identity of M corresponds to nearest Ar (c) ACE Interpretation 0.1 or 0.2 for 1 dp balance, 0.01 or 0.02 for 2 dp balance, 0.001 or 0.002 for 3 dp balance in (i) and 100 (i) of mass water in (ii) (expression or correct answer) (d) ACE Improvement Problem and appropriate improvement required. Not all the water was lost (1) so heat to constant mass (1) Water was regained on cooling (1) so cool in a desiccator/ with a lid (1) Use a balance with more dp (1) this gives a lower percentage error (1) The solid frothed/spat on heating (1) so use a deeper crucible/heat even more gently (1) Small amount of solid used gives high percentage error (1) use more solid. Qn 2 Total [4] [5] [1] [2] [12]Cambridge International Examinations 2014<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_v2_MS_-_Paper_3_CIE_Chemistry_A-level_2.jpg)