Cambridge A Level Chemistry 9701 — 2019 May/June Paper 3 · Variant 4

1 Iron wire contains impurities. You will investigate the percentage by mass of iron in a sample of iron wire. A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron(II) sulfate. Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g) You will titrate the solution of iron(II) sulfate with potassium manganate(VII) of known concentration to determine the amount of iron(II) ions present and hence the percentage by mass of iron in the wire. You may assume the impurities do not form any products that react with potassium manganate(VII). 5Fe2+(aq) + MnO4‒(aq) + 8H+(aq) 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) FB 1 is 0.0200 mol dm‒3 potassium manganate(VII), KMnO4. FB 2 is a solution of FeSO4 prepared by reacting 6.02 g of iron wire with sulfuric acid to make 1 dm3 of solution. FB 3 is dilute sulfuric acid, H2SO4. (a) Method ●● Fill a burette with FB 1. ●● Pipette 25.0 cm3 of FB 2 into a conical flask. ●● Use the measuring cylinder to transfer 25 cm3 of FB 3 into the conical flask. ●● 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 certain that any recorded results show the precision of your practical work. ●● Record all of your burette readings and the volume of FB 1 added in each accurate titration. Keep FB 3 for use in Question 2. I Results II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FB 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 2 required .............................. cm3 of FB 1. [1] (c) (i) ive your answers to (ii), (iii), (iv) and (v) to the appropriate number of significant figures. [1] (ii) Use your answer to (b) to calculate the number of moles of potassium manganate(VII), FB 1, which reacted with 25.0 cm3 of FB 2. moles of MnO4‒ = .............................. mol [1] (iii) Use the information on page 2 to calculate the number of moles of iron(II) ions present in 25.0 cm3 of FB 2. moles of Fe2+ = .............................. mol [1] (iv) Calculate the mass of iron present in 25.0 cm3 of FB 2. mass of Fe = .............................. g [1] (v) Calculate the percentage by mass of iron in the sample of iron wire. percentage by mass of iron in iron wire = .............................. % [1] (d) A student suggested that when a piece of iron wire was dissolved in a known volume and concentration of sulfuric acid, the number of moles of iron that reacted with the acid could be determined by working out how much acid was left after the reaction. The amount of excess acid could be determined by titrating the mixture with a known concentration of sodium hydroxide. Explain whether the student was correct. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 14]

Mark scheme: 1(a) I The following data must be shown • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings 1 II Headings and units correct for accurate titration table and headings match readings. • initial / start (burette) reading / volume + unit • final / end (burette) reading / volume + unit • titre or volume / FB 1 and used / added + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry 1 III All accurate burette readings to 0.05 cm3 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 Award V if 0.30 < δ ⩽ 0.50 cm3 1 Award VI if 0.20 < δ ⩽ 0.30 cm3 1 Award VII if δ ⩽ 0.20 cm3 1 1(b) 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 titres selected. 1 1(c)(i) All final answers to (ii)–(v) to 3 or 4 sf Minimum 3 answers displayed 1 1(c)(ii) Correctly calculates × 0.02 1000 (b) 1 1(c)(iii) Correct use of (ii) × 5 1 1(c)(iv) Correct use of (iii) × 55.8 1 1(c)(v) Correctly uses × × 40 1 00 6.02 (c)(iv) or × 1 00 0.1505 (c)(iv) 1 Question Answer Marks 1(d) Student incorrect sodium hydroxide will also react with Fe2+ / iron(II) / iron(II) sulfate OR Impurity (in wire) reacts with sulfuric acid / sodium hydroxide 1

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2 In Question 1 you used dilute sulfuric acid, FB 3. You will now determine the concentration of FB 3 by measuring the temperature of its reaction with sodium hydroxide. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) FB 4 is 0.90 mol dm‒3 sodium hydroxide, NaOH. (a) Method ●● Fill the second burette with FB 4. ●● Support the plastic cup in the 250 cm3 beaker. ●● Pipette 10.0 cm3 of FB 3 into the plastic cup. ●● Place the thermometer into the FB 3. Tilt the cup so that the bulb of the thermometer is surrounded by solution. Record the temperature in the table below. This is the temperature of the solution before any FB 4 has been added. ●● Add 5.00 cm3 of FB 4 from the burette to the FB 3 in the plastic cup. Stir the mixture and record the temperature reached. Record the volume of FB 4 added. ●● Add a second 5.00 cm3 portion of FB 4 to the plastic cup, stir the mixture and record the temperature reached. Record the total volume of FB 4 added. ●● Continue adding 5.00 cm3 portions of FB 4 until 40.00 cm3 have been added. After each addition, stir and record the temperature reached and total volume of FB 4 added. Results total volume of FB 4 added / cm3 temperature of solution / °C [3] (b) Plot a graph of temperature of solution on the y‑axis against total volume of FB 4 added on the x‑axis. Select a scale on the y‑axis to include a temperature of 2.0 °C above your maximum thermometer reading. Label any points you consider to be anomalous. You will use the graph to find the volume of FB 4 needed to neutralise 10.0 cm3 of FB 3. (c) (i) Use your answer to (b) to calculate the number of moles of sodium hydroxide required to neutralise 10.0 cm3 of FB 3. moles of NaOH = .............................. mol [1] (ii) Calculate the number of moles of sulfuric acid present in 10.0 cm3 of FB 3. moles of H2SO4 = .............................. mol Hence calculate the concentration, in mol dm‒3, of sulfuric acid in FB 3. concentration of H2SO4 in FB 3 = .............................. mol dm‒3 [1] (d) Explain how you would use the data obtained in Question 2 to calculate the enthalpy change of neutralisation of the sulfuric acid. You do not need to carry out the calculation. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [3] [Total: 12] Qualitative Analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen; ● the formation of any precipitate and its solubility in an excess of the reagent added; ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Mark scheme: 2(a) I All 9 specified volumes recorded to nearest 0.05 cm3 and all temperatures to .0 or .5 °C. 1 II Award this mark based on the tolerance table 1 III Award this mark based on the tolerance table 1 Sup ∆Tmax ⩾ 10.5 °C 6.5–10.0 °C 3.5–6.0 °C ⩽ 3.0 °C 1 mark δ = 2.0 °C δ = 1.5 °C δ = 1.0 °C δ = 0.5 °C 2 marks δ = 1.0 °C δ = 1.0 °C δ = 0.5 °C not available 2(b) I Linear scales chosen so that graph occupies more than half the available length for both axes (including extra 2 °C for y-axis). (6 big squares on y-axis & 5 on x-axis) and axes labelled with name or unit 1 II All points recorded accurately plotted (within ½ small square and in the correct square of linear scale within plotting area). 1 III Two best fit straight lines / curves drawn so they intersect / meet – one for increasing temperature and one for decreasing temperature (or no change in temperature). Reject an intersection below the highest temperature recorded unless that point has been labelled as anomalous. 1 IV Correct volume from suitable intersect to 1 or 2 dp A continuous curve cannot score either mark III or IV Neither III nor IV can be scored if there is no max T 1 Question Answer Marks 2(c)(i) Correctly calculates × 0.90 V 1000 (b) to minimum 2 sf 1 2(c)(ii) Correct expression: (c)(i) / 2 and answer × 1000 / 10 1 2(d) Explain how to get m or ∆T using words or figures m = 10 / volume of acid used + volume of NaOH / FB 4 at intersect / at highest temperature / from maximum on graph / needed to neutralise the acid ∆T = T at intersect – initial Tacid / FB 3 (or mean initial Tacid + base) / Tmax – initial T(acid / FB 3) from table Allow quoted correct figures from the table (∆T) or the graph (∆T or m) 1 Use of mc∆T 1 Divide heat energy produced / moles of acid / 1st part of (c)(ii) 1

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3 (a) FB 5 is a solution of a sodium salt of an organic acid. FB 6 and FB 7 each contain one cation and one anion from those listed in the Qualitative Analysis Notes. Carry out the following tests and record your observations. Use a separate 1 cm depth of each solution in a test‑tube for each test. observations test FB 5 FB 6 FB 7 Add 2 or 3 drops of acidified potassium manganate(VII). If there is no visible change, pour into a boiling tube and warm gently. Add a 1 cm depth of dilute nitric acid, then add 2 or 3 drops of aqueous silver nitrate. Add a 1 cm depth of dilute hydrochloric acid, then add a 1 cm depth of aqueous barium nitrate or aqueous barium chloride. Add a 1 cm depth of FB 5 and leave to stand for a few minutes. [7] (b) (i) Select reagents for two further tests to help identify the cations present in FB 6 and FB 7. Record in a suitable form below the reagents you use and your observations. [4] (ii) Use your observations to identify as many ions as possible. Give the formula of the ion present. Write ‘unknown’ if you were unable to identify an ion. FB 6 FB 7 cation anion [2] (iii) Write an ionic equation for any precipitation reaction observed involving FB 7. Include state symbols. ....................................................................................................................................... [1] [Total: 14] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a) Award one mark for every two correct observations (*) On warming with KMnO4 any test that positively identifies a gas is CON for that test apart from with FB 5 (CO2). 7 3(a) test observations FB 5 FB 6 FB 7 + H+ / MnO4‒ no change / no (visible) reaction / stays / turns pink * no change / no (visible) reaction / stays / turns pink / purple and (purple) turns colourless / decolourises / turns yellow * warm Decolourises * no change / stays purple * ignore + HNO3 no change / no (visible) reaction * no change / no (visible) reaction * no change / no (visible) reaction and then Ag+ white ppt * white ppt * no change / no visible reaction / no ppt * + HCl no change / no (visible) reaction / no ppt and no change / no (visible) reaction / no ppt * then Ba2+ no change / no (visible) reaction / no ppt * white ppt * + FB 5 white ppt * (solution) turns yellow * Question Answer Marks 3(b)(i) Selects NaOH and NH3 (allow NH4OH) Ignore a named sulfate (including sulfuric acid) 1 FB 6 FB 7 + NaOH white ppt and insoluble in excess * green ppt and insoluble in excess * + NH3 no (visible) reaction / no change / no ppt * green ppt and insoluble in excess * For two correct observations (*) award one mark Mark vertically or horizontally. 2 FB 7 (ppt) turns brown on standing / in air with either alkali OR No (white) ppt / no reaction stated for FB 6 with the named sulfate. 1 3(b)(ii) FB 6 FB 7 cation Ca2+ / Ba2+ * allow both allow unknown if no sulfate test carried out Fe2+ * anion Cl ‒ SO42‒ * For two correct observations (*) award one mark Allow 1 mark if all correct names given (iron(II)) instead of formulae. 2 Question Answer Marks 3(b)(iii) Precipitation reaction for FB 7: Fe2+(aq) + 2OH‒(aq) → Fe(OH)2(s) or Ba2+(aq) + SO42‒(aq) → BaSO4(s) 1

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