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CIE Jun 2013 v3 Paper 3 Q2
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![Examiners For Use The identity of a metal, M, can be found by titrating a solution of its carbonate with hydrochloric acid of known concentration. FA 3 is a solution of the metal carbonate, M2CO3, of concentration 6.90 g dm3. You are to dilute the hydrochloric acid that you used in Question 1 and then titrate the carbonate solution with this acid. (a) Method Dilution of the acidPipette 25.0 cm3 of FA 1 into the 250 cm3 volumetric (graduated) flask labelled FA 4. Add distilled water to make the total volume 250 cm3. Stopper the flask and mix the contents thoroughly. TitrationFill the burette with diluted hydrochloric acid, FA 4. Use a clean pipette to transfer 25.0 cm3 of FA 3 into a conical flask. Titrate FA 3 with FA 4 using the indicator provided. Perform a rough titration and record your burette readings in the space below. The rough titre iscm3.Carry out as many accurate titrations as you think necessary to obtain consistent results. Make certain any recorded results show the precision of your practical work. Record, in a suitable form below, all of your burette readings and the volume of FA 4 added in each accurate titration. II III IV VI VII [7] [Turn overUCLES 2013 9701/33/M/J/13<br />
Examiners For Use (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 requiredcm3 of FA 4 [1] (c) Calculation The equation for the reaction between hydrochloric acid and the metal carbonate is given below. M2CO3 + 2HCl2MCl + CO2 + H2O (i) Calculate the number of moles of hydrochloric acid present in the volume in (b). (ii) Hence, calculate the number of moles of M2CO3 present in 25.0 cm3 of FA 3. moles of HCl =mol (iii) Calculate the concentration of M2CO3 in FA 3 in mol dm3. moles of M2CO3 =mol concentration of M2CO3 =mol dm3 (iv) Use your answer to (iii), and the fact that FA 3 contains 6.90 g dm3, to determine the relative atomic mass, Ar, of M. (v) Use your answer to (iv) and the Periodic Table on page 16 to suggest the identity of M. Ar of M =M is[5]UCLES 2013 9701/33/M/J/13<br />
Examiners For Use (d) The concentration of a carbonate solution could be found using either the method in Question 1 or that in Question 2. (i) Suggest, and explain, which of the methods is more accurate.(ii) For the method that you think is less accurate, suggest an improvement to the practical procedure that could be made to improve the accuracy.[2] [Total: 15]UCLES 2013 9701/33/M/J/13 [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v3_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 3 Mark Scheme Syllabus Paper GCE AS/A LEVELMay/June 2013 9701 33 (a) PDO Volume given for rough titre and Layout accurate titre details tabulated. (Minimum 22 boxes) PMT MMO II Initial and final burette readings recorded for rough titre and Collection volume of FA 4 added recorded for each accurate titre. Headings and units correct for accurate titration. PDO III All accurate burette readings to 0.05 cm Recording Do not award this mark if: 50(.00) is used as an initial burette reading; more than one final burette reading is 50(.00); any burette reading is greater than 50(.0). MMO Decisions IV Has two uncorrected accurate titres within 0.1 cm Do not award if, having performed two titres within 0.1 , a further titration is performed that is more than 0.1 cm cm from the closer of the original 2 titres unless a further titration has been carried out which is within 0.1 cm of any others. Do not award if titres from burette readings to no dp are used (apart from use of 0 for initial reading). Examiner rounds any accurate burette readings to the nearest 0.05 cm , checks subtractions and then selects the best titres for Supervisor and candidate using the hierarchy: two identical; titres within 0.05 cm to calculate mean correct to 0.01 cm ; titres within 0.1 cm ; etc. Examiner compares candidate mean titre with Supervisor mean titre. MMO Quality V, VI and VII Award V, VI and VII for0.20 cm Award V and VI for 0.20 cm Award V for 0.40 cm <0.60 cm <0.40 cm Apply spread penalty as follows: If best titres are0.50 cm cancel one of the Q marks. [7]Cambridge International Examinations 2013<br />
Page 4 Mark Scheme Syllabus Paper GCE AS/A LEVELMay/June 2013 9701 33 (b) ACE Mean titre is correctly calculated from clearly selected values Interpret- (ticks or working). PMT ation Candidate must average two (or more) titres that are within 0.20 cm of each other. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 eg 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct. eg 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Note: the candidates mean will sometimes be marked as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy. [1] (c) (i) ACE Correctly calculates 0.095(b) to 3 or 4 sf. Interpret- 1000 ation (ii) ACE Correctly calculates (c)(i) to 3 or 4 sf Interpret- ation (iii) and (c)(ii)1000 to 3 or 4 sf (do not penalise sf twice). 25.0 (iv) ACE 6.90/ (c)(ii) 60 Interpret- ation calculated to 02 dp (v) ACE Corresponding identity of M (must be Group 1) (can be from Conclusion negative numberignore sign). PDO Working in the correct direction shown in at least 3 stages in Display (i), (ii), (iii) and (iv). [5]Cambridge International Examinations 2013<br />
PMT Page 5 Mark Scheme Syllabus Paper GCE AS/A LEVELMay/June 2013 9701 33 (d) (i) ACE (Titration more accurate) Interpret- because temperature rises are small or ation titration apparatus/burette/pipette is more accurately calibrated or more precise or lower % error or is more accurate than measuring cylinder (ora for measuring cylinder) or the indicator gives an exact/precise end point but measuring temperature rise does not. (ii) ACE use more volumes near the maximum T or Improve- use burette/pipette or ment better insulation/use of lid or use more accurately calibrated thermometer or increase concentration(s) or measuring initial T of solutions for each expt carried out [2] [Total: 15] FA 5 is ZnCO3 + NaBr; FA 6 is NaNO2; FA 7 is Na2SO4 (b) (i) MMO Effervescence / fizzing / bubbling or gas (evolved) which Collection turns limewater milky. (ii) MMO White precipitate, dissolves in excess sodium hydroxide. Collection ACE Zn , Al and Pb Allow zinc, aluminium, lead no ecf. 2+ 3+ 2+ Conclusion (iii) MMO Suitable pair of reagents chosen to distinguish between the Decisions 3 expected ions (NH3 + one other). PDO Six correct theoretical results for the three ions. Display Allow -- for no reaction Award one mark if one set of theoretical results match the given reagent (ie mark horizontally or vertically) ecf possible from observations in (ii) (for 1 mark) as pairs require a single reagent 2+ 2+ Mg and Ca if white ppt insoluble in excess in (ii); 2+ Ba and NH4 if no ppt obtained in (ii); two out of the correct three ions are chosenCambridge International Examinations 2013<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v3_MS_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
