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CIE Jun 2013 v1 Paper 3 Q2
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![A second way to determine the concentration of an acid is by volumetric titration. In this experiment you willrst dilute the sample of FA 2 that you used in Question 1 and then titrate this diluted solution using aqueous sodium hydroxide. H2SO4(aq) + 2NaOH(aq)Na2SO4(aq) + 2H2O(l) Examiners For Use FA 2 is dilute sulfuric acid, H2SO4. FA 3 is 0.150 mol dm3 sodium hydroxide, NaOH. distilled water (a) Method Dilution of FA 2Use the burette labelled FA 2 to transfer 25.00 cm3 of FA 2 into the 250 cm3 graduated (volumetric) flask, labelled FA 4. Make up the contents of the flask to the 250 cm3 mark with distilled water. Stopper the flask and mix the contents thoroughly. This is solution FA 4.TitrationFill the burette labelled FA 3 with FA 3. Use a clean pipette to transfer 25.0 cm3 of FA 4 into a conical flask. Add to the flask a few drops of the acid-base indicator provided. Titrate the acid in the flask with the alkali, FA 3. You should perform a rough titration. In the space below record your burette readings for this rough titration. The rough titre iscm3. 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 3 added in each accurate titration. Make certain that any recorded results show the precision of your practical work.UCLES 2013 9701/31/M/J/13 II III IV [5] [Turn over<br />
(b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value. 25.0 cm3 of FA 4 requiredcm3 of FA 3. [1] (c) (i) Calculate how many moles of NaOH are contained in the volume recorded in (b). Examiners For Use (ii) Hence, calculate how many moles of H2SO4 are contained in 25.0 cm3 of FA 4. moles of NaOH =mol (iii) Calculate the concentration of the sulfuric acid, FA 2. moles of H2SO4 =mol II III concentration of FA 2 =mol dm3 [3] (d) You have used two methods to determine the concentration of the sulfuric acid in FA 2. Use your answers to 1(d)(iii) and 2(c)(iii) to calculate the difference in these values as a percentage of the value found by the volumetric titration method. percentage difference =% [1] [Total: 10]UCLES 2013 9701/31/M/J/13<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v1_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 3 (e) Mark Scheme GCE AS/A LEVELMay/June 2013 Syllabus 9701 Paper 31 ACE improvements [1] Accuracy of temperature measurementuse a 0 to 50 C thermometer or a thermometer with smaller scale divisions (not just more accurate/ electronic thermometer/parallax). Uncertainty about where the lines crosssample more values of FA 2 in the region of the intersection. Repeat/ extra readings on LHS of intersection/ near maximum. Initial temperatures of acid and alkali not samemeasure both. Other answers acceptable if specific. [Total: 15] 2 (a) MMO collection Initial and final volumes recorded for rough AND initial, final and volume added recorded for accurate titre. PDO recording II All accurate readings recorded to 0.05 cm3. Do not award 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 decision III Two uncorrected accurate titres within 0.1 cm3. Do not award if, having performed 2 titres within 0.1 cm3, a further titration is performed that is >0.1 cm3 from the closer of the original 2 titres unless a further titration has been carried out which is within 0.1 cm3 of any other. MMO quality IV + V Award 2 marks if difference from Supervisor [5] within 0.20 cm3. Award 1 mark if difference from Supervisor within 0.50 cm3. Examiner compares candidate mean titre with Supervisor mean titre. If best titres are0.5 cm3, cancel one of the Q marks.Cambridge International Examinations 2013<br />
Page 4 (b) Mark Scheme GCE AS/A LEVELMay/June 2013 Syllabus 9701 Paper 31 ACE interpretation [1] Calculates the mean to appropriate decimal places. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 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. (c) (d) ACE interpretation All answers correct. (i) 0.15(b) /1000 (ii) (i)/2 (iii) (ii)400 PDO display Working shown in (i) and (iii) PDO display All answers given to 3 or 4 sig figs (minimum 2). [3] ACE interpretation Correctly works out % difference to min 2 sig figs. [1] [Total: 10]Cambridge International Examinations 2013<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v1_MS_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
