Cambridge A Level Chemistry 9701 — 2018 Oct/Nov Paper 3 · Variant 4

1 In this experiment you will determine the percentage by mass of an impure sample of sodium hydrogencarbonate, NaHCO3. You will do this by titration with hydrochloric acid, HCl. The impurity in the sample is X. X is a sodium compound which does not react with HCl. FB 1 is a mixture containing sodium hydrogencarbonate and X. You are supplied with approximately 6.5 g of FB 1. You will also use FB 1 in Question 2. FB 2 is 0.105 mol dm–3 hydrochloric acid, HCl. methyl orange indicator (a) Method Preparing a solution of FB 1 ● Weigh the 100 cm3 beaker. Record the mass. ● Add between 2.8 g and 3.0 g of FB 1 to the beaker. ● Reweigh the beaker with FB 1. Record the mass. ● Calculate and record the mass of FB 1 used. ● Add approximately 50 cm3 of distilled water to FB 1 in the beaker. ● Stir the mixture with a glass rod until all the FB 1 has dissolved. ● Transfer this solution into the 250 cm3 volumetric flask. ● Wash the beaker with distilled water and transfer the washings to the volumetric flask. ● Add distilled water to the volumetric flask up to the mark. ● Shake the flask thoroughly. ● This solution of impure sodium hydrogencarbonate is FB 3. Label the flask FB 3. Titration of FB 3 ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Add approximately 10 drops of methyl orange indicator. ● Carry out a rough titration. ● Record your burette readings and the rough titre 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 VII VIII [8] (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 3 required .............................. cm3 of FB 2. [1] (ii) Calculate the number of moles of hydrochloric acid, HCl, in the volume of FB 2 calculated in (b). moles of HCl = .............................. mol [1] (iii) Complete and balance the equation for the reaction of sodium hydrogencarbonate with hydrochloric acid. Include state symbols. ....NaHCO3...... + ....HCl...... ....NaCl...... + ....CO2...... + ................. Deduce the number of moles of sodium hydrogencarbonate that reacted with the number of moles of HCl calculated in (ii). moles of NaHCO3 = .............................. mol [1] (iv) Use your answer to (iii) to calculate the number of moles of sodium hydrogencarbonate in the FB 1 that you weighed out. moles of NaHCO3 in FB 1 used = .............................. mol [1] (v) Calculate the percentage by mass of NaHCO3 in FB 1. percentage by mass of NaHCO3 in FB 1 = .............................. % [1] [Total: 14] Question 2 starts on the next page.

Mark scheme: 1(a) I Unambiguous data recorded (mass or g) • two weighings with clear indication to what they refer. • (mass of) FB 1, correctly subtracted and between 2.80 and 3.00 1 II All the following data is recorded • rough titration: both burette readings and the titre • initial and final burette readings for two (or more) accurate titrations 1 III Titre values recorded for accurate titrations, and correct headings and units in the accurate titration table • initial / start and (burette) reading / volume (allow vol, but not V) • final / end and (burette) reading / volume • titre or volume / FB 2 and used / added (not ‘difference’ or ‘total volume’) • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded 1 IV All accurate burette readings are recorded to the nearest 0.05 cm3. The requirement to record to 0.05 applies to burette readings, including 0.00 cm3 (if this was the initial reading), but it does not apply to the titre. 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 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Examiner calculates supervisor mean titre and hence the supervisor’s value of mean tire / mass of FB 1. Examiner calculates candidate’s mean titre and hence the candidate’s value of mean tire / mass of FB 1. Examiner compares the two values. Q Award VI, VII and VIII if δ ⩽ 0.10 (cm3g–1) 1 Q Award VI and VII if 0.10 < δ ⩽ 0.30 1 Q Award VI only if 0.30 < δ ⩽ 0.50 1 Question Answer Marks 1(b) Candidate calculates the mean correctly. • 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. 26.667 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 – mean 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. Note: the candidate’s mean will sometimes be marked correct even if it was different from the mean calculated by the Examiner for the purpose of assessing accuracy. 1 1(c)(i) All answers quoted in parts (ii)–(v) are given to 3 or 4 sig fig. A minimum of three answers is required to earn this mark 1 1(c)(ii) Correctly calculates moles of HCl No of moles of HCl used = 0.105 × mean titre / 1000 1 1(c)(iii) Equation with correct state symbols and moles NaHCO3 correct NaHCO3(aq / s) + HCl (aq) → NaCl (aq) + CO2(aq / g) + H2O(l) and no of moles of NaHCO3 = ans in (ii) 1 1(c)(iv) Correctly uses (c)(iii) for moles of NaHCO3 No of moles of NaHCO3 in FB 1 = 10 × answer (iii) 1 1(c)(v) Correctly uses answer (c)(iv) in calculating % by mass of NaHCO3 % by mass = (ans (iv) × 84 / mass of FB1 used in (a)) × 100 84 may be shown as the sum of the Ar values. 1

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2 You will determine the percentage by mass of NaHCO3 in FB 1 again, this time by thermal decomposition. The equation for the thermal decomposition of sodium hydrogencarbonate is shown. 2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g) FB 1 is a mixture containing sodium hydrogencarbonate and an impurity, X. (a) Method ● Weigh a crucible with its lid and record the mass. ● Add between 2.8 g and 3.0 g of FB 1 to the crucible. Weigh the crucible and lid with FB 1 and record the mass. ● Place the crucible on the pipe-clay triangle. Heat the crucible and contents gently for approximately two minutes, with the lid off. ● Then heat strongly for approximately three minutes. ● Replace the lid and leave the crucible and residue to cool for at least five minutes. While the crucible is cooling, you may wish to begin work on Question 3. ● Reweigh the crucible and contents with the lid on. Record the mass. ● Heat the crucible and contents strongly for a further two minutes, without the lid. ● Replace the lid and leave the crucible and residue to cool for at least five minutes. ● Reweigh the crucible and contents with the lid on. Record the mass. ● Calculate and record the starting mass of FB 1 and the mass of residue obtained. I II III IV V [5] (b) Calculations (i) Calculate the number of moles of carbon dioxide produced during the thermal decomposition of FB 1 by using the formula below. mass lost during heating moles of CO2 produced = MrCO2 + MrH2O moles of CO2 produced = .............................. mol [1] (ii) Use your answer to (i) and the equation on page 6 to calculate the mass of sodium hydrogencarbonate in the FB 1 you used in this experiment. mass of NaHCO3 in FB 1 = .............................. g [1] (iii) For this experiment calculate the percentage by mass of NaHCO3 in FB 1. percentage by mass of NaHCO3 in FB 1 = .............................. % [1] (c) (i) Explain why the crucible and contents were heated for a further two minutes after the first weighing of the crucible and residue. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) What assumption did you make about the behaviour of X when you carried out the calculation? ............................................................................................................................................. ....................................................................................................................................... [1] (iii) A student suggested that it would have been more accurate to carry out the thermal decomposition with the lid on the crucible throughout the experiment. State and explain whether or not you agree with this suggestion. ............................................................................................................................................. ....................................................................................................................................... [1] (iv) Suggest which of the two procedures, titration or thermal decomposition, gives a more accurate value for the percentage by mass of NaHCO3 in FB 1. Explain your choice. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 13] 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 Six headings and units in list/table of data: • Mass of crucible and lid (not ‘weight’ in any reading) • Mass of crucible, lid and FB 1 (or ‘contents before heating’) • Mass of crucible, lid and residue / contents after first heating / heating 1 / owtte • Mass of crucible, lid and residue / contents after 2nd heating • Mass of FB 1 (used) • Mass of residue Do not allow ‘mass of FB 1 after heating’ in third or fourth weighing. Unit must be given correctly in each case Inclusion or not of lid must be consistent. 1 II Four weighings recorded, all in the space provided • Four weighings (or more) recorded in the space provided. • All readings recorded to same number of decimal places. • Final weighing is within 0.04 g of the previous one. If balance used read to 1 dp, the 3rd and 4th readings must be the same. 1 III Mass of FB 1 and residue, both written in the space provided • Mass of FB 1 used, correctly subtracted • Mass of FB 1 used was between 2.8 and 3.0 g • Mass of residue correctly subtracted. 1 Accuracy marks • For assessment of accuracy, examiner must check and correct (if necessary) the masses of FB 1 used and of residue obtained by the supervisor and by the candidate. • Work out ratio mass of FB 1 / mass of residue (from lower mass weighing) for the supervisor (2 dp) • Work out ratio (mass FB 1: mass residue) for candidate (2 dp) • Calculate δ, the difference between these two ratios. Award IV and V if δ ⩽ 0.05 Award IV if 0.05 < δ ⩽ 0.10 2 Question Answer Marks 2(b)(i) Correctly calculates moles of CO2 • No of moles CO2 = mass lost / 62 • Answer must be correct and expressed to 2–4 sig fig. Mass loss must be correctly subtracted, either using original weighings or using the masses of FB1 and residue. 1 2(b)(ii) Correctly uses (i) for mass of NaHCO3 Mass of NaHCO3 = ans (i) × 2 × 84 and answer to 2–4 sig fig Do not penalise sf more than once in 2(b) 1 2(b)(iii) Correctly calculates percentage mass of NaHCO3 % = ans (ii) / mass of FB1 used × 100 and answer to 2–4 sig fig. 1 2(c)(i) To obtain constant mass (owtte) This shows that the reaction / decomposition is complete / only Na2CO3 is left 1 Allow: to remove all CO2 and H2O 1 2(c)(ii) (Assumption is that) the impurity does not decompose when heated or The impurity does not react with air when heated or The impurity does not react with NaHCO3/ Na2CO3 on heating 1 2(c)(iii) Disagree because there is no frothing / spitting out (owtte) 1 2(c)(iv) Possible answers, stating and explaining which experiment is more accurate: • Titration, because repeated titres are consistent. • Gravimetric, because fewer measurements / fewer cumulative / combined errors • Gravimetric, because heating is to constant mass. • Gravimetric, because colour change (of methyl orange) is indistinct / gradual. 1

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3 (a) FB 4 has the same composition as the residue obtained in Question 2. It contains two sodium compounds, one of which is X. Both anions present in FB 4 are listed in the Qualitative Analysis Notes. (i) To a small spatula measure of FB 4 in a test-tube, add dilute nitric acid slowly. Record your observations. Keep the solution produced for use in (ii). ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) To a 1 cm depth of the solution obtained in (i) in a test‑tube, add a few drops of aqueous silver nitrate, followed by aqueous ammonia. Record your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Give the equation for one reaction taking place in (i). State symbols are not required. ....................................................................................................................................... [1] (b) Dissolve the remaining FB 4 in a 5 cm depth of distilled water in a boiling tube. This solution is FB 5. (i) Carry out the following tests and record your observations. Use a 1 cm depth of FB 5 in a test-tube for each test. test observations Add several drops of aqueous copper(II) sulfate, then add dilute sulfuric acid. Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute nitric acid. Add a few drops of methyl orange indicator. Add several drops of aqueous silver nitrate, then add dilute nitric acid. [6] (ii) To a 1 cm depth of FB 5 in a boiling tube, add an equal volume of aqueous sodium hydroxide and warm carefully, then add a small piece of aluminium foil to the mixture. Record your observations. [1] (iii) Using your observations in (a) and (b), name X. ....................................................................................................................................... [1] (iv) What can you deduce about FB 4 from the observation when methyl orange indicator was added to FB 5? ....................................................................................................................................... [1] [Total: 13] 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)(i) Fizzing / effervescence / bubbling 1 Gas / CO2 turns lime water milky / cloudy white or gas gives white precipitate with lime water 1 3(a)(ii) White ppt with AgNO3 and ppt is soluble in (excess) ammonia / colourless solution formed. 1 3(a)(iii) Either equation correct: Na2CO3 + 2HNO3 → 2NaNO3 + CO2 + H2O or Ca(OH)2 + CO2 → CaCO3 + H2O 1 3(b)(i) Blue precipitate (with CuSO4) and (pale) (not dark) blue solution (formed with H2SO4) 1 Fizzing / effervescence / bubbling with acid in both this test and addition of acid to BaCO3 ppt in next test. 1 White precipitate (with Ba2+) and precipitate dissolves / colourless solution formed (with HNO3) 1 Yellow / orange-yellow / yellowy-orange (solution) (formed) 1 White / off-white precipitate (with Ag+) 1 Fizzes / bubbles / effervescence (with HNO3) or ppt is partially soluble (in HNO3) (owtte) 1 3(b)(ii) Fizzing / effervescence / bubbling with aluminium 1 3(b)(iii) Sodium chloride / NaCl 1 3(b)(iv) Mark ecf on colour Yellow: alkaline / neutral / weakly acidic Orange: neutral / weakly acidic 1

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