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CIE Jun 2015 v4 Paper 3 Q1
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![The concentration of aqueous ammonia used in qualitative analysis is 2 mol dm3 but it is supplied in a much more concentrated form. This is referred to as .880 ammonia. You are to determine the concentration of .880 ammonia by titration of a solution of ammonia, FB 1, with hydrochloric acid of known concentration. The equation for the reaction is given below. NH3(aq) + HCl (aq)NH4Cl (aq) FB 1 is a dilute solution of ammonia, NH3(aq). It was prepared by measuring out 5.91 cm3 of the .880 ammonia and then adding distilled water until the solution had a volume of 1 dm3. FB 2 is 0.100 mol dm3 hydrochloric acid, HCl (aq). methyl orange indicator (a) MethodFill the burette with FB 2. Use the pipette to transfer 25.0 cm3 of FB 1 into a conical flask. Add a few drops of methyl orange indicator. 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 achieve 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 FB 2 added in each accurate titration. II III IV VI VII [7] (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 FB 1 requiredcm3 of FB 2. [1]UCLES 2015 9701/34/M/J/15<br />
(c) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Calculate the number of moles of hydrochloric acid present in the volume of FB 2 calculated in (b). (ii) Use your answer to (i) to determine the number of moles of ammonia present in 25.0 cm3 of FB 1, pipetted into the conicalask. moles of HCl =mol (iii) Use your answer to (ii) to calculate the concentration, in mol dm3, of the diluted ammonia, FB 1. moles of NH3 =mol concentration of NH3 (diluted) in FB 1 =mol dm3 (iv) Use your answer to (iii) and the information on page 2 to calculate the concentration, in mol dm3, of .880 ammonia. concentration of .880 ammonia =mol dm3 [3] (d) A student analysed a different sample of concentrated ammonia and determined the concentration to be 15.0 mol dm3. Calculate the percentage difference in concentration of the .880 ammonia you have determined compared with that of the student. (If you have been unable to complete the calculation, assume the concentration of .880 ammonia was 9.35 mol dm3. This is not the correct value.) percentage difference in concentration =% [1]UCLES 2015 9701/34/M/J/15 [Total: 12] [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v4_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Question (a) (i) (ii) (iii) (iv) Cambridge International AS/A LevelMay/June 2015 9701 34 Mark Scheme Syllabus Paper Indicative material Mark Total Initial and final burette readings and titre unambiguously recorded in rough and accurate titrations. (22 accurate boxes) Headings and units correct for accurate titration and headings match readings. Initial / final (burette) reading / volume or Reading / volume at start / finish and Titre or Volume / and Units: (cm3) or / cm3 or in cm3 or cm3 by every entry FB 2 added / used [not difference] All accurate burette readings 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.(00). Two accurate titres within 0.10 cm3. Do not award if 3rd titre > 0.10 cm3 away from any other unless a further titration is also carried out which is within 0.1 cm3 of any other. Do not award the mark if any accurate burette readings (apart from initial 0) are given to zero dp. Examiner checks and corrects titre subtractions where necessary. Examiner selects the best mean titre using a hierarchy: two identical titres within 0.05 cm3, two or more titres within 0.10 cm3 etc. Examiner subtracts (corrected) candidates titre from Supervisors titre. (b) 1 1 1 [7] [1] Award V, VI and VII if< 0.20 cm3. Award V and VI if 0.20 << 0.40 cm3. Award V if 0.40 < <0.60 cm3. Spread penalty: if the two best titres are0.50 cm3 apart, cancel one of the Q marks Candidate must average two (or more) titres that are all within 0.20 cm3. 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. 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. e.g. 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)(i)(ii) (answer to (b)0.1) / 1000 and (ii) = (i) and answer to 3 or 4 sfCambridge International Examinations 2015<br />
Page 3 Question Cambridge International AS/A LevelMay/June 2015 9701 34 Mark Scheme Syllabus Paper Indicative material Mark Total (iii) (c)(ii)1000 / 25 and answer expressed to 3 or 4 sf. (iv) Correct expression is shown (c)(iii) 1000 / 5.91 [3] [1] [12] (d) Total (a) 15(c)(iv)100 / 15 or 15(c)(iv)100 / (c)(iv) (Table of) results with unambiguous headings (not weight) showing4 balance readings 4 thermometer readings 2mass FB 3 added 2T II All balance readings recorded to the same number of dp (at least one dp) and thermometer readings to .0 or .5 C (minimum 2 of each) III Masses of FB 3 used between the limits given in the question (2.9(0) to 3.1(0) g and 4.9(0) to 5.1(0) g) Examiner corrects thermometer readings to the nearest 0.5 C, checks subtractions then calculates T1m2 / where T1 is the smaller temperature fall. T2m1 Award IV if ratio falls between 0.85 and 1.15 Award IV and V if ratio falls between 0.95 and 1.05 (b) (i) mean mass correctly calculated from weighings to 2 dp (ii) mean temperature fall correctly calculated from thermometer (c) (i) readings to 1 dp Do not award if values used for calculating mean are from incorrect subtractions. Correctly calculates (b)(i) / 53.5 [5] [2] 1 1Cambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v4_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
