Cambridge A Level Chemistry 9701 — 2017 Oct/Nov Paper 3 · Variant 6

1 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. 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 dm–3 potassium manganate(VII), KMnO4. FB 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. (a) Method ● Fill 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 is ............................. cm3. ● 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. I II III IV V VI [6] (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. 25.0 cm3 of FB 1 required ............................. cm3 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). moles of MnO4– = ............................. mol (ii) Calculate the total number of moles of ethanedioate ions present in 25.0 cm3 of FB 1. total moles of (COO–)2 = ............................. mol [2] 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 dm–3 sodium hydroxide, NaOH. thymol blue indicator (e) Method ● Fill 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 is ............................. cm3. ● 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. [4] (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. 25.0 cm3 of FB 1 required ............................. cm3 of FB 4. (ii) Calculate the number of moles of sodium hydroxide in the volume of FB 4 calculated in (i). moles of NaOH = ............................. mol (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 (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. moles of (COONa)2 = ............................. mol (ii) Calculate the mass of sodium ethanedioate present in 25.0 cm3 of FB 1. mass of (COONa)2 = ............................. g (iii) Use your answer to (f)(iii) to calculate the mass of ethanedioic acid present in 25.0 cm3 of FB 1. mass of (COOH)2 = ............................. g (iv) Calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FB 1. 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]

Mark scheme: 1(a) I 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. 2 × 2 box). Headings and units are not required for this mark 1 II Headings and units correct for accurate titration table and headings match readings. • initial / start (burette) reading / volume • final / end (burette) reading / volume • titre or volume / FB 2 and used / added • units: (cm3) or / cm3 or in cm3 or cm3 for each volume recorded 1 III 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). 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 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 • accurate titres within 0.05 cm3, then • accurate 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 candidate’s mean titre value with that of the Supervisor. Question Answer Marks 1(a) Award V and VI if δ ⩽ 0.30 (cm3) 1 Award V if 0.30 < δ ⩽ 0.60 cm3 1 1(b) 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. (e.g. 26.666 cm3 must be rounded to 26.67 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. (e.g. 26.0 and 26.2 = 26.1 is allowed) (e.g. 26.0 and 26.1 = 26.1 is wrong – should be 26.05) 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. • All burette readings used to calculate the mean were recorded as integers 1 1(c)(i) Correctly calculates number of moles of MnO4 – × = 0.02 vol in 1000 (b) 1 1(c)(ii) Correctly uses: (i) × 5/2 (to a minimum of 2 sf) 1 Question Answer Marks 1(d) (COOH)2(aq) + 2NaOH(aq) → (COONa)2(aq) + 2H2O(l) 1 1(e) Table clearly showing 2 (or more) accurate initial and final volumes and titres. Subtraction for titres must be correct. 1 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 • accurate titres within 0.05 cm3, then • accurate 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 candidate’s mean titre value with that of the Supervisor. Award 3 marks if δ ⩽ 0.20 cm3. 1 Award 2 marks if 0.20 < δ ⩽ 0.40 cm3. 1 Award 1 mark if 0.40 < δ ⩽ 0.60 cm3. 1 1(f)(i), (ii) and (iii) Correctly uses mean in (i) and number of moles of NaOH × = 0.04 vol in 1000 (i) in (ii) and moles (COOH)2 = (ii)/2 in (iii) 1 1(g)(i) Correctly uses (c)(ii) – (f)(iii) 1 1(g)(ii) Correctly uses mass (COONa)2 = (g)(i) × 134 1 1(g)(iii) Correctly uses mass (COOH)2 = (f)(iii) × 90 1 Question Answer Marks 1(g)(iv) Correct expression (g)(iii) (g)(ii) + (g)(iii) × 100 (or correct answer) 1 Significant figures mark Answers to (c), (f) and (g) all to 3 or 4 sf (Minimum 6 answers attempted) 1 1(h)(i) No change • since 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 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 1(i) Would be more accurate since the titre volume is bigger so smaller percentage error. 1

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2 Qualitative Analysis At each stage of any test you are to record details of the following: ● colour changes seen; ● the formation of any precipitate; ● the solubility of such precipitates in an excess of the reagent added. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. Where gases are released they should be identified by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. No additional tests for ions present should be attempted. If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible. FB 5, FB 6 and FB 7 are aqueous solutions that each have an ion containing one of the metals from those listed in the Qualitative Analysis Notes. (a) Carry out the following tests and record your observations. test observations (i) To a 1 cm depth of FB 5 in a test-tube add a 1 cm depth of aqueous sodium hydroxide, then add several drops of hydrogen peroxide. (ii) To a 1 cm depth of FB 6 in a test-tube add aqueous sodium hydroxide. (iii) To a 1 cm depth of FB 6 in a test-tube add several drops of hydrogen peroxide and then add aqueous sodium hydroxide. (iv) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of dilute sulfuric acid and then add a few drops of FB 7. (v) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of FB 7. (vi) To a 1 cm depth of aqueous potassium iodide in a test-tube add a few drops of FB 7, then add a few drops of aqueous starch. [8] (b) Identify the metal present in FB 5, FB 6 and FB 7. FB 5 contains ................................................ FB 6 contains ................................................ FB 7 contains ................................................ [3] (c) What do your observations in (a)(vi) tell you about what has happened to the iodide ions on addition of FB 7 to KI(aq)? You may give your answer in the form of an equation. .............................................................................................................................................. [1] (d) (i) FB 8 is a solid sample of the compound present in aqueous solution FB 7. Heat all of FB 8 in a hard-glass test-tube gently for about 10 s and then strongly for about 20 s. observations ........................................................................................................................ ............................................................................................................................................. (ii) Leave the test-tube and contents to cool completely. To the cooled test-tube add a 1 cm depth of aqueous sodium hydroxide. Observe the appearance of the contents of the test-tube. appearance ......................................................................................................................... [2] (e) FB 6 contains one of the anions Cl –, Br –, I–, SO42– or SO32–. (i) Construct a table to show reagents you would use to identify which anion is present in FB 6. Include in your table space to record your observations and deductions. (ii) Carry out your tests on FB 6 until you have identified the anion. Record your observations and deductions in your table. anion in FB 6 = .............................. [3] [Total: 17]

Mark scheme: 2(a)(i) sodium hydroxide buff / pale or light brown / fawn / beige / off-white ppt 1 hydrogen peroxide (turns) dark brown / black solid / ppt 1 effervescence / bubbling / fizz and gas relights glowing splint 1 2(a)(ii) green ppt insoluble in excess 1 2(a)(iii) brown ppt insoluble in excess 1 2(a)(iv) purple to colourless (allow purple to (pale) yellow / pale orange) 1 2(a)(v) brown solid / ppt 1 2(a)(vi) yellow / brown (solution) and blue-black / black / dark blue with starch 1 2(b) FB 5 manganese / Mn / Mn2+ / Mn(II) 1 FB 6 iron / Fe / Fe2+ / Fe(II) 1 FB 7 manganese / Mn / Mn(VII) 1 2(c) (iodide ions are) oxidised to iodine / (iodide ions) lose electrons to form iodine / 2I– → I2 + 2e(–) 1 2(d)(i) Gas relights glowing splint 1 2(d)(ii) Green solution / liquid 1 Question Answer Marks 2(e)(i) Tests: (aqueous) silver nitrate and (aqueous) barium nitrate / chloride 1 white ppt with Ba2+ and insoluble in named acid (not sulfuric acid) and no ppt with Ag+ 1 2(e)(ii) anion = sulfate / SO4 2– 1

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