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CHEMISTRY Advanced Practical Skills Candidates answer on the Question Paper. Additional Materials: As listed in the Instructions to Supervisors
October/November 2010 2 hours
READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Give details of the practical session and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. You may lose marks if you do not show your working or if you do not use appropriate units. Use of a Data Booklet is unnecessary. Qualitative Analysis Notes are printed on pages 11 and 12. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Laboratory
For Examiner’s Use 1 2 3 Total
This document consists of 11 printed pages and 1 blank page. DC (CW/SW) 28271/4 © UCLES 2010
2 There are three questions on this paper. Question 2 should not be the last question attempted. 1 You are to determine the concentration of hydrochloric acid, which supplies the H+ ions in the following reaction. For Examiner’s Use
IO3–(aq) + 5I–(aq) + 6H+(aq)
3H2O(l) + 3I2(aq)
In the presence of an excess of IO3– ions and an excess of I– ions, the amount of I2 liberated is directly proportional to the amount of H+ ions present and can be determined by titration with sodium thiosulfate, Na2S2O3. You are provided with the following reactants. FA 1 hydrochloric acid FA 2 containing 15.0 g dm–3 sodium thiosulfate, Na2S2O3.5H2O aqueous potassium iodate(V), KIO3 aqueous potassium iodide, KI (a) Method • • • Fill a burette with FA 2. Pipette 25.0 cm3 of FA 1 into the conical flask. Use a 25 cm3 measuring cylinder to add to the flask 10 cm3 of aqueous potassium iodate(V) and 10 cm3 of aqueous potassium iodide. There is an excess of each of these reagents. Place the flask on a white tile. Titrate the liberated iodine with FA 2. During the titration the colour of the iodine in the solution will fade from red-brown to orange to yellow. The end-point occurs when the solution just goes colourless with the addition of a single drop of FA 2. You should perform a rough titration. In the space below record your burette readings for this rough titration.
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I II The rough titre is ................................. cm3 Carry out as many accurate titrations as you think necessary to obtain consistent results. Record in a suitable form below all of your burette readings and the volume of FA 2 added in each accurate titration. Make certain any recorded results show the precision of your practical work. III IV V VI VII
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© UCLES 2010 9701/31/O/N/10
3 (b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value. For Examiner’s Use
25.0 cm3 of FA 1 require .............. cm3 of FA 2.  Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (c) (i) Calculate the concentration, in mol dm–3, of the sodium thiosulfate in FA 2. FA 2 contains 15.0 g dm–3 Na2S2O3.5H2O. [Ar: H, 1.0; O, 16.0; Na, 23.0; S, 32.1]
The concentration of sodium thiosulfate in FA 2 is ............................ mol dm–3. (ii) Calculate how many moles of Na2S2O3 are contained in the volume of FA 2 recorded in (b).
.............. mol of Na2S2O3 (iii) Calculate how many moles of iodine, I2 reacted with the Na2S2O3 in (ii). 2Na2S2O3(aq) + I2(aq) Na2S4O6(aq) + 2NaI(aq)
............................ mol of iodine...