CIE Nov 2017 v6 Paper 3 Q1 (with explained solutions) |

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CIE Nov 2017 v6 Paper 3 Q1

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FB 1 is a solution made by dissolving an unknown mass of a mixture of ethanedioic acid, (COOH)2, and sodium ethanedioate, (COONa)2. You will carry out two titrations to find the percentage by mass of ethanedioic acid in the mixture. PMT Titration 1 In aqueous solution both ethanedioic acid and sodium ethanedioate release all their ethanedioate ions, (COO)2. These ions react with manganate(VII) ions as shown. 2MnO4 (aq) + 16H+(aq) + 5(COO)2(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l) FB 1 is an aqueous solution of the mixture containing ethanedioic acid and sodium ethanedioate. FB 2 is 0.0200 mol dm3 potassium manganate(VII), KMnO4. FB 3 is 1.00 mol dm3 sulfuric acid, H2SO4. (a) MethodFill a burette with FB 2. Pipette 25.0 cm3 of FB 1 into a conical flask. Use the measuring cylinder to add 30 cm3 of FB 3 to the same conical flask. Place the conical flask on the tripod and gauze and heat until the solution is at a temperature of approximately 70 C. Carefully remove the flask from the tripod and place it under the burette, ready for the titration. Add FB 2 from the burette, slowly at first, until a permanent pale pink colour is formed. If the reaction mixture turns brown, reheat it to about 70 C. If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration. 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 sure 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 [6]UCLES 2017 9701/36/O/N/17<br />
(b) From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations. Show clearly how you obtained this value. PMT 25.0 cm3 of FB 1 requiredcm3 of FB 2. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of manganate(VII) ions in the volume of FB 2 calculated in (b). (ii) Calculate the total number of moles of ethanedioate ions present in 25.0 cm3 of FB 1. moles of MnO4=mol total moles of (COO)2 =mol [2]UCLES 2017 9701/36/O/N/17 [Turn over<br />
PMT Titration 2 Ethanedioic acid reacts with aqueous sodium hydroxide. In this reaction both the H+ ions formed by the acid molecule react. (d) Complete the equation showing the reaction between ethanedioic acid and sodium hydroxide including state symbols. (COOH)2(aq) + NaOH(aq)+[1] FB 4 is 0.0400 mol dm3 sodium hydroxide, NaOH. thymol blue indicator (e) MethodFill the second burette with FB 4. Pipette 25.0 cm3 of FB 1 into a conical flask. Add about 10 drops of thymol blue indicator. Add FB 4 from the burette until the end-point has been reached. 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 sure 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 4 added in each accurate titration.UCLES 2017 9701/36/O/N/17 [4]<br />
PMT (f) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) From your accurate titration results, obtain a suitable value for the volume of FB 4 to be used in your calculations. (ii) Calculate the number of moles of sodium hydroxide in the volume of FB 4 calculated in (i). 25.0 cm3 of FB 1 requiredcm3 of FB 4. (iii) Use your equation from (d) to calculate the number of moles of ethanedioic acid present in 25.0 cm3 of FB 1. moles of NaOH =mol moles of (COOH)2 =mol [1] (g) (i) Use your answers to (c)(ii) and (f)(iii) to calculate the number of moles of sodium ethanedioate, (COONa)2, present in 25.0 cm3 of FB 1. (ii) Calculate the mass of sodium ethanedioate present in 25.0 cm3 of FB 1. moles of (COONa)2 =mol mass of (COONa)2 =gUCLES 2017 9701/36/O/N/17 [Turn over<br />
(iii) Use your answer to (f)(iii) to calculate the mass of ethanedioic acid present in 25.0 cm3 of FB 1. PMT (iv) Calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FB 1. mass of (COOH)2 =g percentage by mass of (COOH)2 =% [5] (h) A student checked the formula of ethanedioic acid on the internet and found it to be (COOH)2.2H2O. This differs from the formula (COOH)2 that you used in your calculations. The FB 1 you used was made from (COOH)2.2H2O and sodium ethanedioate. State and explain the effect this knowledge has on; (i) the volume of FB 4 needed for reaction in (e),(ii) the calculated percentage by mass of (COOH)2 in the solid mixture used to prepare FB 1.[2] (i) Another student suggested that the investigation could be improved by making the titrations more accurate. He said that the concentrations of FB 2 and FB 4 should be reduced. State and explain whether or not this suggestion would make the titrations more accurate.[1] [Total: 23]UCLES 2017 9701/36/O/N/17<br />

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Cambridge International AS/A LevelMark Scheme PUBLISHED Answer October/November 2017 Marks All the following data is recorded burette readings and titre for rough titration initial and final burette readings for two (or more) accurate titrations (i.e. 22 box). IHeadings and units are not required for this mark II Headings and units correct for accurate titration table and headings match readings.units: (cm3) or / cm3 or in cm3 or cm3 for each volume recorded initial / start (burette) reading / volume final / end (burette) reading / volume titre or volume / FB 2 and used / added III All accurate burette readings to 0.05 cm3. Do not award this mark if:more than one final burette reading is 50.(00);50(.00) is used as an initial burette reading; any burette reading is greater than 50.(00). IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. Examiner rounds any accurate burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best titres using the hierarchy: identical titres thenaccurate titres within 0.05 cm3, thenaccurate titres within 0.10 cm3, etc. These best titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Examiner compares candidates mean titre value with that of the Supervisor. 9701/36 Question 1(a)UCLES 2017 Page 2 of 7<br />
9701/36 Question 1(a) 1(b) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer if0.30 (cm3) Award V and VI Award V if 0.30 <0.60 cm3 October/November 2017 Marks (e.g. 26.666 cm3 must be rounded to 26.67 cm3) Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3.Working / explanation must be shown or ticks must be put next to the two (or more) accurate readings selected.The mean should be quoted to 2 dp, and be rounded to nearest 0.01 cm3. Two special cases, where the mean need not be to 2 dp:Allow mean expressed to 3 dp only for 0.025 or 0.075 (e.g. 26.325 cm3)Allow mean if expressed to 1 dp, if all accurate burette readings were given to 1 dp and the mean is exactly correct. Do not award this mark if:The rough titre was used to calculate the mean.The candidate did only one accurate titration.Burette readings were incorrectly subtracted to obtain any of the accurate titre values.(e.g. 26.0 and 26.2 = 26.1 is allowed) (e.g. 26.0 and 26.1 = 26.1 is wrongshould be 26.05) All burette readings used to calculate the mean were recorded as integers 1(c)(i) Correctly calculatesnumber of moles of MnO4 = 0.02 vol in1000 (b) 1(c)(ii) Correctly uses: (i)5/2 (to a minimum of 2 sf)UCLES 2017 Page 3 of 7<br />
Cambridge International AS/A LevelMark Scheme PUBLISHED Answer October/November 2017 Marks (COOH)2(aq) + 2NaOH(aq)(COONa)2(aq) + 2H2O(l) Table clearly showing 2 (or more) accurate initial and final volumes and titres. Subtraction for titres must be correct. Examiner rounds any accurate burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best titres using the hierarchy: identical titres then These best titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Examiner compares candidates mean titre value with that of the Supervisor.accurate titres within 0.05 cm3, thenaccurate titres within 0.10 cm3, etc. Award 3 marks if0.20 cm3. Award 2 marks if 0.20 <0.40 cm3. Award 1 mark if 0.40 <0.60 cm3. Correctly uses mean in (i) and number of moles of NaOH and moles (COOH)2 = (ii)/2 in (iii) = 0.04 vol in1000 (i) in (ii) 9701/36 Question 1(d) 1(e) 1(f)(i), (ii) and (iii) 1(g)(i) 1(g)(ii) 1(g)(iii) Correctly uses (c)(ii)(f)(iii) Correctly uses mass (COONa)2 = (g)(i)134 Correctly uses mass (COOH)2 = (f)(iii)90UCLES 2017 Page 4 of 7<br />
Cambridge International AS/A LevelMark Scheme PUBLISHED Answer October/November 2017 Marks 9701/36 Question 1(g)(iv) 1(h)(i) Correct expression (g)(iii) (g)(ii) + (g)(iii)100 (or correct answer) Significant figures mark Answers to (c), (f) and (g) all to 3 or 4 sf (Minimum 6 answers attempted) No changesince the number of moles of acid stays the same, or as the water will not react, or as the mole ratio stays the same, or as the concentration of acid (FB 1) stays the same 1(h)(ii) % mass of acid decreases as there is now water as part of the total mass or Mr of (hydrated) acid increases so multiply moles by bigger number so % mass of (hydrated) acid increases 1(i) Would be more accurate since the titre volume is bigger so smaller percentage error.UCLES 2017 Page 5 of 7<br />

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