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CIE Jun 2013 v4 Paper 3 Q2
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![A second way to determine the concentration of an alkali is by volumetric titration. In this experiment you willrst dilute the sample of FB 2 that you used in Question 1 and then titrate the diluted solution using hydrochloric acid. HCl (aq) + NaOH(aq)NaCl (aq) + H2O(l) Examiners For Use FB 2 is aqueous sodium hydroxide, NaOH. FB 3 is 0.200 mol dm3 hydrochloric acid, HCl. (a) Method Dilution of FB 2Use the burette labelled FB 2 to transfer 25.00 cm3 of FB 2 into the 250 cm3 graduated (volumetric) flask, labelled FB 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 solution FB 4. TitrationRinse the unlabelled burette thoroughly with distilled water and then with a little FB 3. Fill this burette with FB 3. Use a pipette to transfer 25.0 cm3 of FB 4 into a conical flask. Add to the flask a few drops of the acid-base indicator provided. Titrate the alkali in the flask with the acid, FB 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 FB 3 added in each accurate titration. Make certain that any recorded results show the precision of your practical work.UCLES 2013 9701/34/M/J/13 II III IV [5] [Turn over<br />
Examiners For Use (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 FB 4 requiredcm3 of FB 3. [1] (c) (i) Calculate how many moles of HCl are contained in the volume recorded in (b). (ii) Hence, calculate how many moles of NaOH are contained in 25.0 cm3 of FB 4. moles of HCl =mol (iii) Calculate the concentration of the sodium hydroxide, FB 2. moles of NaOH =mol II III concentration of FB 2 =mol dm3 [3] [Total: 9]UCLES 2013 9701/34/M/J/13<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v4_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 3 Mark Scheme GCE AS/A LEVELMay/June 2013 Syllabus 9701 Paper 34 [3] [2] [Total: 16] (d) (c) (i) and (ii) ACE interpret- ation I Correctly calculates .and same answer for (c)(ii) (iii) II Correctly calculates. PDO Display ACE interpret- ation III Show use of 2(b)(iii)/1000 in (i) and all 3 answers to 3 or 4 sf Any two of:change in volume makes no difference to the accuracy as temp rise the same decreased accuracy as less accurate measurement of volume with reference to measuring cylinder or burette (comparative needed or reference to precision/calibration or % error)more accurate as greater number of experiments so more points to get an accurate intercept or better lines of best fit (a) MMO collection Initial and final volumes recorded for rough and initial, final and volume added recorded for accurate titrations. PDO recording MMO decision II All accurate burette readings recorded to 0.05 cm3. 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). III Two uncorrected accurate titres within 0.1 cm3. Do not award if, having performed two titres within 0.1 cm3, a further titration is performed that is more than 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 others. Do not award if titres from burette readings to no dp are used (apart from use of 0 for initial reading).Cambridge International Examinations 2013<br />
Page 4 Mark Scheme GCE AS/A LEVELMay/June 2013 Syllabus 9701 Paper 34 Examiner rounds any accurate burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best titres for Supervisor and candidate using the hierarchy: two identical; titres within 0.05 cm3; titres within 0.1 cm3; etc. to calculate mean correct to 0.01 cm3. Examiner compares candidate mean titre with Supervisor mean titre. (a) (cont.) MMO quality IV + V Award 2 marks if0.20 cm3. Award 1 mark if 0.20 <0.50 cm3. (b) ACE interpret- ation If best titres are0.50 cm3, cancel one of the Q marks. Check mean titre is correctly calculated from clearly selected values (ticks or working). Candidate must average two (or more) titres that are within 0.20 cm3 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. (c) ACE interpret- ation (i) Correctly calculates 0.2(b)/1000 and same ans in (ii) to 3 or 4 sf II (c)(ii)400 PDO display III Working in the correct direction shown in (i) and (iii). [5] [1] [3] [Total: 9]Cambridge International Examinations 2013<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_v4_MS_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
