Cambridge A Level Chemistry 9701 — 2025 May/June Paper 3 · Variant 1

1 You will determine the percentage purity of a sample of calcium carbonate by reacting it with excess hydrochloric acid and measuring the volume of carbon dioxide produced. CaCO3 (s) + 2HCl (aq) CaCl 2(aq) + H2O(l) + CO2(g) FA 1 is impure calcium carbonate, CaCO3. FA 2 is 0.500 mol dm–3 hydrochloric acid, HCl . (a) Method • Weigh the container with FA 1. Record the mass in the space for results. • Fill the tub with water to a depth of approximately 5 cm. • Fill the 250 cm3 measuring cylinder completely with water. Holding 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 the open end is in the water just above the base of the tub. • Using the 50 cm3 measuring cylinder, transfer 50.0 cm3 of FA 2 into the flask labelled X. Check that the bung fits tightly into 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 the flask. Tip all of the FA 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. • Replace the flask in the clamp and leave until the fizzing stops. Swirl the flask occasionally. • Weigh the container with any residual FA 1. Record the mass. • Calculate and record the mass of FA 1 that is added to the acid. • When no more gas is collected, record the final volume of gas. You may wish to start Question 2 or Question 3 while the gas is being collected. Results I II III [3] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder (at room conditions). amount of CO2 = .............................. mol Hence, deduce the amount, in mol, of calcium carbonate present in the FA 1 used. amount of CaCO3 = .............................. mol [1] (ii) Use your answer to (b)(i) and the mass of FA 1 used to calculate the percentage purity of calcium carbonate. Show your working. purity of calcium carbonate = .............................. % [1] (c) A student carries out the experiment described in (a) using 25.0 cm3 of 1.00 mol dm–3 hydrochloric acid. The mass of FA 1 is not changed. Suggest the consequence of this change on the percentage purity of calcium carbonate calculated by placing one tick (3) in Table 1.1. Table 1.1 The percentage purity calculated would increase. The percentage purity calculated would stay the same. The percentage purity calculated would decrease. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [2] (d) The student repeats the method in (a) but the hydrochloric acid is not used in excess. (i) State an observation that shows that the acid is not used in excess. ..................................................................................................................................... [1] (ii) Explain what effect, if any, this would have on the percentage purity of calcium carbonate calculated in (b)(ii). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 10]

Mark scheme: Question Answer Marks 1(a) I Unambiguous headings in space for results AND correct units in the space for results for: 1 • (mass of) container with FA 1 / solid • (mass of) container (+ residual FA 1 / solid) • (mass of) FA 1 • (volume of) gas / CO2 (produced) II Both weighings to a consistent number of decimal places (either to 2 dp or to 3 dp) 1 AND mass of FA 1 correctly calculated AND volume of gas (in cm3) produced recorded as an integer value (No headings are required for this mark.) III Volume of gas produced 1 Award in range 90–210 cm3 (0.090–0.210 dm3) 1(b)(i) Correctly calculates 1 (amount of CO2 =) volume of gas collected / 24000 mol AND amount of CaCO3 = amount of CO2 AND both answers to 2–4 sf 1(b)(ii) Correct display 1 % purity = [(b)(i) x 100.1/mass of FA 1]  100 AND answer to 2–4 sf 1(c) M1: the bottom box is ticked 2 AND the volume of gas collected is less M2: more gas escapes (before the bung is replaced) AND the (initial) rate is greater 1(d)(i) One of: 1 • unreacted solid / FA 1 / CaCO3 remains in the (conical) flask (after bubbling stops) • not all the solid / FA 1 has reacted / disappeared (at end of reaction) 1(d)(ii) % purity is less AND 2 M1: volume of gas collected is less % purity is less AND M2: so amount / moles / mass of calcium carbonate (calculated) is lower

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2 You will determine the percentage purity of another sample of calcium carbonate, CaCO3, by titration. The experiment involves three steps. step 1 A known mass of the same impure calcium carbonate, FA 1, is reacted with an excess of hydrochloric acid to form FA 3. This step has been done for you. step 2 You will dilute the products of step 1 to a known volume. step 3 You will carry out a titration to find out how much acid remains after the reaction in step 1. FA 3 has been prepared by reacting 19.0 g of FA 1 with 250 cm3 of 2.00 mol dm–3 hydrochloric acid, HCl . FA 5 is 0.0900 mol dm–3 sodium hydroxide, NaOH. FA 6 is bromophenol blue indicator. (a) Method step 2 • Pipette 25.0 cm3 of FA 3 into the 250 cm3 volumetric flask. • Make the solution up to 250 cm3 with distilled water. • Thoroughly mix the contents of the volumetric flask. This solution is FA 4. step 3 • Fill the burette with FA 5. • Rinse the pipette with distilled water and then with FA 4. • Pipette 25.0 cm3 of FA 4 into a conical flask. • Add several drops of FA 6. • 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 in the space below, all your burette readings and the volume of FA 5 added in each accurate titration. Results I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FA 4 required .............................. cm3 of FA 5. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium hydroxide in the volume of FA 5 in (b). amount of NaOH = .............................. mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid in 250 cm3 of FA 4. amount of HCl in 250 cm3 of FA 4 = .............................. mol Hence, calculate the amount, in mol, of hydrochloric acid present in 250 cm3 of FA 3. amount of HCl in 250 cm3 of FA 3 = .............................. mol [1] (iv) Calculate the amount, in mol, of hydrochloric acid used to prepare FA 3. amount of HCl = .............................. mol [1] (v) Use your answers to (c)(iii) and (c)(iv) to calculate the amount, in mol, of hydrochloric acid that reacted with the FA 1 in step 1. amount of HCl that reacted with FA 1 = .............................. mol [1] (vi) Use your answer to (c)(v) to calculate the amount, in mol, of calcium carbonate present in FA 1. CaCO3 (s) + 2HCl (aq) CaCl 2(aq) + H2O(l) + CO2(g) amount of CaCO3 in FA 1 = .............................. mol [1] (vii) Use your answer to (c)(vi) and the mass of FA 1 used to calculate the percentage purity of calcium carbonate. Show your working. purity of calcium carbonate = .............................. % [1] (d) The percentage purity of calcium carbonate in FA 1 has been determined by two different methods, gas collection and titration. The gas collection method used in Question 1 is less accurate. (i) Suggest two reasons why the gas collection method is less accurate. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (ii) Describe a change to the gas collection method that would improve the accuracy of the percentage purity determined in Question 1. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 18] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: • colour changes seen • the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added • the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I The following data must be shown 7 • two burette readings AND titre for rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume / FA 5 AND used / added unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded III All accurate burette readings are to nearest 0.05 cm3 IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. Accuracy (Q) marks: marks V, VI and VII will be determined by comparison of candidate and supervisor Round burette readings to the nearest 0.05 cm3. Check and correct titre subtractions where necessary. Select the best mean titre, using the following hierarchy: • 2 identical accurate titres (ignoring any that are labelled ‘rough’), then • accurate titres within 0.05 cm3, then • accurate titres within 0.10 cm3, etc. Calculate the Supervisor’s mean titre to two decimal places. Calculate the candidate’s mean titre to two decimal places. Calculate the difference () between the supervisor’s and candidate’s mean titre. V, VI, VII Award V if  ⩽ 0.80 cm3 Award VI if  ⩽ 0.50 cm3 Award VII if  ⩽ 0.30 cm3 2(b) Correct calculation of the mean titre 1 • 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 must be quoted to 2 decimal place and be rounded to the nearest 0.01 cm3, e.g., 26.675 must be rounded to 26.68. 2(c)(i) Significant figures 1 All quoted answers in (c)(ii)–(c)(v) are expressed to 3 or 4 sf. 2(c)(ii) Correctly calculates 1 amount of NaOH = 0.09  (vol (b)/1000) mol 2(c)(iii) Correctly calculates 1 amount of HCl in 250 cm3 FA 4 = (c)(ii)  10 mol AND amount of HCl in 250 cm3 FA 3 = [(c)(ii)  10]  10 mol 2(c)(iv) Correctly calculates 1 amount of HCl used to prepare FA 3 = (250/1000)  2 = 0.500 mol 2(c)(v) Correctly uses (c)(iv) – (c)(iii) 1 2(c)(vi) Correctly uses 1 amount of CaCO3 present = (c)(v)/2 mol 2(c)(vii) Correctly uses 1 % purity = [(c)(vi)  100.1 / 19.0]  100 2(d)(i) Any two of: 2 • some gas / carbon dioxide dissolves in the water (in trough / measuring cylinder) • some gas escapes before the bung is replaced on the conical flask • the impurity may react to produce gas 2(d)(ii) Any one of: 1 • description of both CaCO3 and acid being separated within the stoppered flask until reaction starts, e.g. use of small container for CaCO3 which can be tipped over, package of CaCO3 held by a thread, etc. • saturate the water with CO2 prior to experiment / use of warmer water • use of gas syringe

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3 (a) FA 7 is a solution containing two cations and one anion. All of the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Tables 3.1 and 3.2. For each test use a 1 cm depth of FA 7 in a test-tube unless the solution requires heating. Table 3.1 test observations Test 1 Add aqueous ammonia. Test 2 To a 1 cm depth of FA 7 in a boiling tube, add aqueous sodium hydroxide, then warm the mixture carefully. Test 3 Add a few drops of silver nitrate. Table 3.2 test observations Test 4 Add a 1 cm depth of aqueous barium chloride or aqueous barium nitrate. Test 5 Add a 1 cm depth of dilute nitric acid, then leave to stand for a few minutes. Test 6 Add a 1 cm depth of aqueous potassium iodide, then add a few drops of starch solution. [5] (ii) Write an ionic equation for a reaction in (a)(i) that resulted in the formation of a precipitate. Include state symbols. ..................................................................................................................................... [1] (iii) Give the formula of each ion present in FA 7. If you cannot identify an ion, write ‘unknown.’ cations ..................... and ..................... anion ..................... [2] (b) FA 8 is an anhydrous sodium compound. (i) Transfer a spatula measure of FA 8 to a hard-glass test-tube. Heat the test-tube gently at first, then more strongly until no further change occurs. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) The FA 9 provided is a sample of the residue obtained from heating FA 8. To a 2 cm depth of nitric acid in a test-tube, slowly add a small spatula measure of FA 9. Record your observations. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Deduce the formula of FA 8. FA 8 is ......................... . [1] [Total: 12]

Mark scheme: 3(a)(i) 2 * = 1 mark (round down) 5 Test 1 (pale) blue ppt * soluble in excess * (to give a) dark / deep blue solution * NH3(aq) Test 2 (pale) blue ppt AND insoluble in excess * NaOH(aq) warm gas / bubbles / NH3 turns (red) litmus blue * black solid / ppt / residue * Test 3 white ppt * AgNO3 Test 4 no (visible) change / remains pale blue solution / no (visible) reaction / no Ba2+(aq) ppt * Test 5 no (visible) change / remains pale blue solution / no (visible) reaction / no Dilute HNO3 effervescence AND standing no change / remains pale blue solution * Test 6 Brown * KI(aq) starch Blue-black / dark blue / black * 3(a)(ii) Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) 1 OR Ag+(aq) + Cl–(aq) → AgCl(s) OR 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq) 3(a)(iii) Cations: Cu2+, NH4+ 2 Anions: Cl– 2 correct = 1 mark. All 3 correct = 2 marks 3(b)(i) condensation / water vapour / (colourless) droplets / steam 1 3(b)(ii) M1: effervescence / bubbling / fizzing 2 M2: EITHER gas / fizzing / CO2 gives a white ppt with limewater OR (white solid / FA 9 reacts and) colourless solution formed 3(b)(iii) FA 8 is NaHCO3 1 from appropriate observations: condensation in (b)(i) AND evidence of CO2 in (b)(ii)

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