Cambridge A Level Chemistry 9701 — 2019 Oct/Nov Paper 3 · Variant 5

1 In this experiment you will determine the concentration of a sample of hydrochloric acid. You will do this by measuring the volume of hydrogen produced when an excess of magnesium reacts with the acid. Mg(s) + 2HCl (aq) MgCl 2(aq) + H2(g) FA 1 is magnesium powder, Mg. FA 2 is hydrochloric acid, HCl. (a) Method ● Weigh the container with FA 1. Record the mass. ● Fill the tub with water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so that the open end is just above the base of the tub. ● Use the 25 cm3 measuring cylinder to place 25.0 cm3 of FA 2 into the reaction flask, labelled X. ● Check that the bung fits tightly in the neck of flask X, clamp flask X, and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of flask X. Tip all of FA 1 into flask X and replace the bung immediately. Remove the flask from the clamp and swirl to mix the contents. ● Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp. ● Measure and record the final volume of gas in the measuring cylinder. ● Weigh and record the mass of the container with any residual solid. ● Calculate and record the mass of FA 1 used. Keep FA 2 for use in Question 2. [2] (b) Calculations (i) Calculate the number of moles of hydrogen gas produced. (Assume 1 mol of gas occupies 24.0 dm3 at this temperature.) moles of H2(g) = .............................. mol [1] (ii) Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = .............................. mol dm–3 [1] (iii) In this experiment the magnesium powder was in excess. Calculate the mass of magnesium powder needed for complete reaction with all the hydrochloric acid in 25.0 cm3 of FA 2. mass of Mg = .............................. g [1] (c) A student suggested two modifications to the method in (a) to give a more accurate value for the concentration. For each suggestion, state whether you agree with the student and explain your answer. Suggestion 1: Use magnesium ribbon rather than powdered magnesium; keep the rest of the experiment the same. .................................................................................................................................................... .................................................................................................................................................... Suggestion 2: Use twice the mass of magnesium powder; keep the rest of the experiment the same. .................................................................................................................................................... .................................................................................................................................................... [2] (d) Another student carried out the experiment in (a) but used less magnesium than that calculated in (b)(iii). State and explain the effect this would have on the calculated concentration of hydrochloric acid in FA 2. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 8]

Mark scheme: 1(a) 3 masses recorded with unambiguous headings in the space provided, with correct units mass used correctly calculated volume of gas collected or final volume recorded with correct units 1 Award this mark if volume recorded by candidate lies within ± 10% of supervisor value. 1 1(b)(i) Correctly calculates volume of gas in cm3 / 24 000 answer to 2–4 sf 1 1(b)(ii) Correct use of: 2 × AND ans (b)(i) / 0.025 (answer to 2–4 sf) 1 1(b)(iii) Correctly uses ans (b)(i) × 24.3 and answer to 2–4 sf 1 1(c) Student correct as reaction now slower so less gas lost (while bung is being fitted). 1 Student incorrect as Mg is in excess. or Student incorrect as reaction is faster so more gas lost 1 1(d) gas volume / amount / moles lower so concentration is lower 1

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2 In this experiment you will determine the concentration of FA 2 by titration using aqueous sodium hydroxide. HCl (aq) + NaOH(aq) NaCl (aq) + H2O(l) FA 2 is hydrochloric acid, HCl. FA 3 is 0.100 mol dm–3 sodium hydroxide, NaOH. methyl orange indicator (a) Method Dilution of FA 2 ● Fill the burette with FA 2. ● Run between 40.00 and 45.00 cm3 from the burette into the 250 cm3 volumetric flask. ● Record the volume used. ● Make the solution up to the 250 cm3 mark by adding distilled water. ● Shake the flask thoroughly to ensure mixing. ● Label this solution of hydrochloric acid FA 4. volume of FA 2 used = .............................. cm3 Titration ● Rinse the burette with distilled water and then with a little FA 4. ● Fill the burette with FA 4. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add several drops of methyl orange indicator. ● Perform a rough titration and record your burette readings. 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 all of your burette readings and the volume of FA 4 added in each accurate titration. I II III IV V VI VII V I [8] (b) From your accurate titration results, obtain a value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required .............................. cm3 of FA 4. [1] (c) Calculations (i) Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of hydrochloric acid that reacted with 25.0 cm3 of FA 3. moles of HCl = .............................. mol [1] (iii) Calculate the concentration of hydrochloric acid in FA 4. concentration of HCl in FA 4 = .............................. mol dm–3 [1] (iv) Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = .............................. mol dm–3 [1] (d) Calculate the maximum percentage error in the volume of FA 2 you added to the volumetric flask. maximum percentage error = .............................. % [1] (e) In Question 1 and Question 2 you have determined the concentration of FA 2 by two different methods. Each method used has possible sources of error, for example in Question 1 the largest source of error is escape of gas. Apart from this error, state and explain a source of error for each method. Question 1 ................................................................................................................................. .................................................................................................................................................... Question 2 ................................................................................................................................. .................................................................................................................................................... [2] [Total: 16] 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 Uses a volume between 40.00 and 45.00 cm3 and answer to at least 1 dp 1 II The following data must be shown • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings 1 III Headings and units correct for accurate titration table and headings match readings. • Initial / start (burette) and reading / volume + unit • Final / end (burette) and reading / volume + unit • titre or volume / FA 4 and used / added (not ‘difference’ amount or ‘total’) + unit 1 IV All accurate burette readings to 0.05 cm3 1 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award VI if 20 < δ ⩽ 30 cm3 1 Award VII if 10 < δ ⩽ 20 cm3 1 Award VIII if δ ⩽ 10 cm3 1 2(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 2(c)(i) Answers for (ii), (iii) and (iv) given to 3–4 sf. Minimum three answers displayed. 1 2(c)(ii) Correctly calculates 2.50 × 10–3 1 2(c)(iii) Correct use of ans (c)(ii) × 1000 / ans (b) 1 2(c)(iv) Correct expression: ans (c)(iii) × 250 / vol used from (a) 1 2(d) Correctly calculates 0.10 / vol used in (a) × 100. 1 Question Answer Marks 2(e) Question 1 • measuring cylinder greater error than burette / pipette • molar gas volume of 24 dm3 may not be valid / temperature of the lab may not be known • too much gas for the measuring cylinder (check that vol > 250 cm3) • use gas syringe (if volume < 100 cm3) 1 Question 2 • dilution introduces extra stage / greater cumulative error • methyl orange end-point can be difficult to see / colour change gradual / difficult to see 1

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3 (a) FA 5 is a salt that contains two different cations and a single anion from those listed in the Qualitative Analysis Notes. (i) Place a small spatula measure of FA 5 in a hard-glass test-tube and heat gently. Do not inhale the fumes. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Pour a 4 cm depth of distilled water into a boiling tube. Add the remaining FA 5 and stir carefully until the solid has dissolved. This solution is FA 6. Carry out the following tests on FA 6 and record your observations. test observations To a 1 cm depth in a test‑tube, add aqueous ammonia. To a 1 cm depth in a boiling tube, add aqueous sodium hydroxide, then warm the mixture. test observations To a 1 cm depth in a test‑tube, add aqueous barium nitrate or aqueous barium chloride, then add dilute hydrochloric acid or dilute nitric acid. [4] (iii) Identify the three ions in FA 5. FA 5 contains .............................. , .............................. and .............................. . [2] (b) A student carried out Qualitative Analysis tests on a hydrated salt, FA 7, and concluded that it contained the ions K+, Cr3+ and SO42–. The relative formula mass of FA 7 is 499.3. Determine the formula of FA 7. The formula of FA 7 is ........................................................... . [2] Question 3 continues on page 10. (c) FA 8 is a solution containing a single cation and a single anion, both of which are listed in the Qualitative Analysis Notes. (i) Carry out the following tests and record your observations. test observations To a 1 cm depth in a test‑tube, add a few drops of aqueous acidified potassium manganate(VII), then add starch indicator. To a 1 cm depth in a test‑tube, add aqueous sodium hydroxide. [2] (ii) Identify the two ions in FA 8. FA 8 contains ...................................................... and ...................................................... . [1] (iii) Suggest an additional test you could carry out to confirm the presence of the anion in FA 8. Carry out this test and record your result. [2] (iv) Give the ionic equation for the reaction you carried out using FA 8 and sodium hydroxide. Include state symbols. ....................................................................................................................................... [1] [Total: 16]

Mark scheme: 3(a)(i) melts / dissolves • condensation / moisture on the walls of the test-tube / steam produced • white smoke / fumes (NOT gas) • (gas) turns red litmus blue • gas turns blue litmus red • white residue Award 1 mark for two correct observations from the list, award 2 marks for three or more correct observations. If both gas observations are given they must be in the correct order for both to be credited. 2 3(a)(ii) NH3 White ppt and insoluble in excess 1 NaOH White ppt and sol in excess Allow 1 mark for white ppt with both NH3 and NaOH 1 hot NaOH Gas / NH3 (on warming) turns red litmus blue 1 Ba2+ White ppt insoluble in acid / white ppt no reaction with acid Reject white ppt formed when acid added 1 3(a)(iii) names or correct formulae 1 NH4+ , Al3+ , SO42– Award 1 mark for two ions, award 2 marks for three ions. 1 Question Answer Marks 3(b) Any formula (involving all three ions) in which the charges on the ions cancel (e.g. K3Cr(SO4)3) 1 KCr(SO4)2.12H2O 1 3(c)(i) Red-brown (allow yellow / yellow-brown / orange / orange-brown / brown) (solution) or KMnO4 / purple decolourises and turns blue-black / dark blue / black (on adding starch) 1 Green ppt and insoluble in excess / turns brown (on standing) Reject grey-green 1 3(c)(ii) Fe2+ / iron(II) and I– / iodide This mark is free-standing 1 3(c)(iii) Uses silver nitrate and yellow ppt 1 ppt insoluble in HNO3 or ppt insoluble in NH3 (nitric acid may be added initially) 1 3(c)(iv) Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s) ecf for Cr3+ / Cu2+ / Fe3+ or any other transition metal ion concluded in (ii). 1

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