Cambridge IGCSE Chemistry (9-1) 0971 — 2018 May/June Paper 6 · Variant 1

1 The volume of dilute nitric acid that reacts with 25.0 cm3 of aqueous potassium hydroxide can be found by titration using the apparatus shown. ........................................... dilute nitric acid 25.0 cm3 of aqueous potassium hydroxide with indicator (a) Complete the box to name the apparatus. [1] (b) Name a suitable indicator that could be used. .............................................................................................................................................. [1] A student did the titration four times and recorded the following results. titration number volume of dilute nitric acid / cm3 1 18.1 2 18.9 3 18.3 4 18.2 (c) (i) Which one of the results is anomalous? ....................................................................................................................................... [1] (ii) Suggest what might have caused this result to be anomalous. ....................................................................................................................................... [1] (iii) Use the other results to calculate the average volume of dilute nitric acid that reacted with the aqueous potassium hydroxide. ....................................................................................................................................... [2] (d) The equation for the reaction taking place in the titration is shown. HNO3 + KOH KNO3 + H2O The student concluded that the aqueous potassium hydroxide was more concentrated than the dilute nitric acid. Explain whether or not the student’s conclusion was correct. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 8]

Mark scheme: 1(a) burette 1 1(b) methyl orange / thymolphthalein / litmus 1 1(c)(i) number 2 / 18.9 cm3 1 1(c)(ii) overshot end point / more than 25 cm3 KOH in flask 1 1(c)(iii) 18.2 1 cm3 1 1(d) (wrong conclusion) nitric acid more concentrated / stronger 1 smaller volume of acid needed 1

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2 A student investigated the rate of reaction between dilute hydrochloric acid and aqueous sodium thiosulfate. When these chemicals react they form a precipitate which makes the solution go cloudy. The formation of this precipitate can be used to show how fast the reaction proceeds. Five experiments were done using the apparatus shown. eye 10 cm3 of dilute hydrochloric acid 250 cm3 conical flask aqueous sodium thiosulfate printed sheet of paper printed words Experiment 1 ● A large measuring cylinder was used to pour 50 cm3 of aqueous sodium thiosulfate into a 250 cm3 conical flask. The conical flask was placed on a printed sheet of paper. ● 10 cm3 of dilute hydrochloric acid was added to the solution in the conical flask. A timer was started immediately and the mixture was swirled. ● The time taken for the printed words to disappear from view was measured. Experiment 2 ● The large measuring cylinder was used to pour 40 cm3 of aqueous sodium thiosulfate into a conical flask, followed by 10 cm3 of distilled water. The conical flask was placed on the printed sheet of paper. ● 10 cm3 of dilute hydrochloric acid was added to the solution in the conical flask. The timer was started immediately and the mixture was swirled. ● The time taken for the printed words to disappear from view was measured. Experiment 3 ● Experiment 2 was repeated but using 35 cm3 of aqueous sodium thiosulfate and 15 cm3 of distilled water. Experiment 4 ● Experiment 2 was repeated but using 30 cm3 of aqueous sodium thiosulfate and 20 cm3 of distilled water. Experiment 5 ● Experiment 2 was repeated but using 10 cm3 of aqueous sodium thiosulfate and 40 cm3 of distilled water. (a) Record the volumes of distilled water used in the table. Use the stop-clock diagrams to record the results in the table. time taken for volume of volume of the printed wordsexperiment aqueous sodium distilled water stop-clock diagram to disappear thiosulfate / cm3 / cm3 from view / s 0 seconds 0 1 50 45 15 5 15 10 minutes 30 0 0 2 40 45 15 5 15 10 30 0 0 3 35 45 15 5 15 10 30 0 0 4 30 45 15 5 15 10 30 0 0 5 10 45 15 5 15 10 30 [3] (b) Plot the results from Experiments 1–5 on the grid. Draw a smooth line graph. 240 210 180 150 time taken for the printed words to disappear from view / s 120 90 60 30 0 0 10 20 30 40 50 volume of aqueous sodium thiosulfate / cm3 [3] (c) (i) From your graph, deduce the time taken for the printed words to disappear from view if Experiment 2 were repeated using 20 cm3 of aqueous sodium thiosulfate and 30 cm3 of distilled water. Show clearly on the grid how you worked out your answer. .............................. s [2] (ii) The rate of reaction can be calculated using the equation shown. 1 rate of reaction = time taken Calculate the rate of reaction using your answer from (c)(i). ....................................................................................................................................... [1] (d) (i) In which experiment, 1, 2, 3, 4 or 5, was the rate of reaction greatest? ....................................................................................................................................... [1] (ii) Explain, in terms of particles, why the rate of reaction was greatest in this experiment. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (e) Give the name of a more accurate piece of apparatus for measuring volumes than a measuring cylinder. .............................................................................................................................................. [1] (f) Suggest the effect on the results of using a 100 cm3 conical flask instead of a 250 cm3 conical flask. Explain your answer. .................................................................................................................................................... .............................................................................................................................................. [2] (g) Sketch on the grid the graph you would expect if all of the experiments were repeated at a lower temperature. Clearly label your graph. [1] [Total: 16]

Mark scheme: 2(a) volume boxes completed correctly in seconds 0, 10, 15, 20, 40 1 time boxes completed correctly 27, 33, 45, 66, 201 2 2(b) all points plotted correctly (± half a small square) 2 smooth line graph 1 2(c)(i) value from graph 1 with clear indication 1 2(c)(ii) 1 ÷ value from (c)(i) 1 2(d)(i) experiment 1 1 Question Answer Marks 2(d)(ii) more particles of thiosulfate (in a given volume) 1 more chance of collision 1 2(e) use a pipette / burette 1 2(f) times would be shorter 1 idea of depth of solution is greater 1 2(g) sketch curve roughly same shape and above original 1

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3 Two substances, solution A and solid B, were analysed. tests on solution A Some of the tests and observations are shown. tests on solution A observations Solution A was divided into three equal portions in three test-tubes. test 1 The pH of the first portion of solution A was pH = 1 tested. test 2 Magnesium ribbon was added to the second effervescence portion of solution A. The gas produced was tested. gas ‘popped’ with a lighted splint test 3 Dilute nitric acid and aqueous barium nitrate white precipitate formed were added to the third portion of solution A. (a) Identify the gas produced in test 2. .............................................................................................................................................. [1] (b) Identify solution A. .............................................................................................................................................. [2] tests on solid B Solid B was zinc carbonate. Complete the expected observations. (c) Dilute nitric acid was added to solid B. The gas produced was tested. observations ............................................................................................................................... .............................................................................................................................................. [2] The zinc nitrate solution formed in the test in (c) was divided into two portions in two test‑tubes. (d) (i) Drops of aqueous sodium hydroxide were added to the first portion of the zinc nitrate solution. observations .................................................................................................................. [2] (ii) An excess of aqueous sodium hydroxide was then added to the mixture. observations .................................................................................................................. [1] (e) (i) Drops of aqueous ammonia were added to the second portion of the zinc nitrate solution. observations .................................................................................................................. [1] (ii) An excess of aqueous ammonia was then added to the mixture. observations .................................................................................................................. [1] [Total: 10]

Mark scheme: 3(a) hydrogen / H2 1 3(b) sulfuric 1 acid 1 3(c) limewater 1 milky / cloudy / white ppt. 1 3(d)(i) white 1 precipitate 1 3(d)(ii) dissolves / clears / goes colourless 1 3(e)(i) white precipitate 1 3(e)(ii) dissolves / clears / goes colourless 1

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4 Potassium chloride is a salt that dissolves in water. The solubility of a salt is the mass in grams of the salt that dissolves in 100 cm3 of water at a particular temperature. Plan an investigation to determine the solubility of potassium chloride in water at 40 °C. You are provided with potassium chloride and common laboratory apparatus. ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ..................................................................................................................................................... [6] [Total: 6]

Mark scheme: 4 any 6 from one method: evaporation • measured volume of water • using measuring cylinder / pipette / burette • heat to 40 °C / heat to >40 °C • add KCl until no more dissolves / add excess KCl • stir • filter mixture (if heated to >40 °C then need to cool and filter) • evaporate filtrate to dryness • weigh solid mass not used • measured volume of water • using measuring cylinder / pipette / burette • heat to 40 °C • add KCl until no more dissolves • stir • weigh KCl not added • weigh KCl before adding any to water – only awarded if weighed mass not used after • difference in mass of KCl is mass dissolved mass undissolved • measured volume of water • using measuring cylinder / pipette / burette • heat to 40 °C • stir • filter • weigh residue (do not award if residue washed) • add weighed (excess) KCl to water – only awarded if mass of residue measured • mass KCl dissolved = initial mass – final mass max 6

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