Chemistry 9701/53 May/June 2024: question paper and mark scheme
Cambridge AS & A Level Chemistry (9701), Paper 5 Planning, Analysis and Evaluation, May/June 2024. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.
Titration can be used to determine the concentration of dissolved oxygen in samples of river water.
The procedure for the experiment is given.
step 1 Use five 50 cm3 graduated syringes, A, B, C, D and E, to collect five separate 30.0 cm3 samples of river water.
step 2 In the laboratory, carefully add 5.0 cm3 of 0.220 mol dm–3 manganese(II) sulfate,
MnSO4(aq), into syringe A and mix well.
step 3 Add 5.0 cm3 of alkaline aqueous potassium iodide into syringe A and mix well.
step 4 Add 10.0 cm3 of dilute sulfuric acid into syringe A and mix well.
step 5 Transfer the contents of syringe A into a 150 cm3 conical flask. Rinse syringe A using
10 cm3 of distilled water and add washings to the conical flask.
step 6 Carry out one accurate titration of all the contents in the conical flask with 0.00200 mol dm–3 aqueous sodium thiosulfate, Na2S2O3(aq), using starch indicator.
Repeat steps 2–6 for the samples in syringes B–E.
(a) Aqueous sodium thiosulfate can be prepared from Na2S2O3•5H2O(s).
(i) Determine the mass, in g, of Na2S2O3•5H2O(s) required to prepare 500.0 cm3 of
0.00200 mol dm–3 Na2S2O3(aq).
mass of Na2S2O3•5H2O(s) = …… g [1]
(ii) Identify the piece of apparatus that should be used to prepare 500.0 cm3 of
0.00200 mol dm–3 Na2S2O3(aq) after the required mass of Na2S2O3•5H2O(s) has been weighed out.
…… [1]
(b) The graduations on each syringe are every 1.0 cm3.
(i) Calculate the percentage error in the measurement of 5.0 cm3 of alkaline aqueous potassium iodide by the syringe.
Show your working.
percentage error = …… [1]
(ii) Place one tick (✓) in each row in Table 1.1 to show the effect, if any, of using a larger volume of alkaline aqueous potassium iodide.
Table 1.1
greater effect no effect smaller effect uncertainty of the measurement percentage error of the measurement
[1]
(c) The sample in the conical flask and the prepared solution of sodium thiosulfate are provided.
Describe the following procedures for the experiment using syringe A.
(i) Preparing the clean burette before taking any readings.
…… [2]
(ii) Carrying out the one accurate titration in step 6.
…… [2]
(d) Suggest why the reaction mixture is mixed well in steps 2–4.
…… [1]
(e) Draw a table for recording the titration results for the five samples in syringes A–E.
[2]
(f) The overall reaction taking place in the experiment is shown.
O2(aq) + 4S2O32–(aq) + 4H+(aq) 2S4O62–(aq) + 2H2O(l)
A student carries out the experiment and determines the mean titre to be 12.65 cm3.
Calculate the concentration, in mol dm–3, of dissolved oxygen in the river water.
concentration of dissolved oxygen = …… mol dm–3 [2]
(g) Freshly distilled water does not contain any dissolved oxygen.
A student decides to run the procedure on a sample of freshly distilled water and at the end obtains a value of 2.26 × 10–5 mol dm–3 dissolved oxygen.
(i) Suggest why the student did not get a value of 0 mol dm–3. Assume the procedure was carried out correctly.
…… [1]
(ii) Suggest how the value of 2.26 × 10–5 mol dm–3 could be used to improve the answer in (f).
…… [1]
(h) Suggest why this method is unsuitable for samples of tap water that have been purified by chlorination and so contain Cl2(aq).
…… [1]
[Total: 16]
Mark scheme
1(a)(i) 0.248(2) (g) 1
1(a)(ii) (500 cm3) volumetric flask 1
1(b)(i) 2 × 0.5 1
×100 = 20(.0%)
5.0
correct working must be shown
1(b)(ii) greater no effect smaller 1
uncertainty of the
Π
measurement
percentage error of the
Π
measurement
1(c)(i) M1 rinse / wash (50 cm3 burette) with (0.0200 mol dm–3 aqueous) sodium thiosulfate (before filling with aqueous sodium 2
thiosulfate)
M2 place (aqueous) sodium thiosulfate in the burette (to fill) AND then run some solution out (through the jet) (by opening
the tap)
1(b)(ii) M1 2
add / run Na2S2O3(aq) (into the conical flask)
until (permanent) colour change (seen) / end-point reached
M2 add dropwise (towards the end) (to ensure the end-point is accurate)
1(d) to ensure the reaction(s) is / are complete (in the syringe) 1
1(e) 2
(titration) A (titration) B (titration) C (titration) D (titration) E
final (burette)
reading / cm3
initial (burette)
reading / cm3
titre / cm3
M1 headings and units correct for table
initial / start and (burette) reading / volume
final / end and (burette) reading / volume
titre or volume / (0.002(00) mol dm–3) sodium thiosulfate (solution) and used / added
units: (cm3) or / cm3 or in cm3 for all headings
M2 five columns for syringes A–E
1(f) M1 n(Na2S2O3) = 12.65 / 1000 × 0.00200 = 2.53 × 10–5 2
M2 n(O2) = M1 / 4 = (2.53 × 10–5 / 4 = 6.325 × 10–6 mol)
concentration of O2 = (M1 / 4) × 1000 / 30 (= 2.10833 × 10–4 mol dm–3) (= 2.11 × 10–4 mol dm–3)
1(g)(i) there was (a small amount of dissolved) oxygen in the other solutions / reagents / reactants (used in the experiment) 1
1(g)(ii) subtract the result (2.26 × 10–5 mol dm–3) of the distilled water experiment from the final result for dissolved oxygen in 1(f) 1
1(h) chlorine is an oxidising agent 1
OR chlorine reacts in the same way as oxygen
OR chlorine reacts with Mn2+ / S2O32– / I– / reactant(s)
The activation energy, EA, for the reaction between dilute hydrochloric acid, HCl (aq), and aqueous sodium thiosulfate, Na2S2O3(aq), can be determined by an initial rates method.
2HCl (aq) + Na2S2O3(aq) 2NaCl (aq) + H2O(l) + S(s) + SO2(g)
The solid sulfur formed is seen as a white suspension in the reaction mixture. The reactants are mixed and the time, t, for a fixed quantity of sulfur to be formed is recorded.
1
A measure of the initial rate of the reaction is . t
Standard solutions of 0.100 mol dm–3 Na2S2O3(aq) and 0.500 mol dm–3 HCl (aq) are supplied.
Measurements are taken for a series of temperatures using the following procedure.
step 1 A thermostatically controlled water bath is set up.
step 2 A 100 cm3 conical flask is labelled A and a second 100 cm3 conical flask is labelled B.
step 3 10.00 cm3 of 0.100 mol dm–3 Na2S2O3(aq) is added to flask A. Flask A is placed in the water bath.
step 4 10 cm3 of 0.500 mol dm–3 HCl (aq) is added to flask B. Flask B is placed in the same water bath.
step 5 Wait for 10 minutes.
step 6 Flask A is removed from the water bath and placed on a tile marked with a black cross.
step 7 The contents of flask B are added to flask A and a timer started.
step 8 The timer is stopped when the black cross is no longer visible. The time is recorded.
(a) Suggest a reason why it is necessary to wait for 10 minutes in step 5.
…… [1]
(b) The procedure does not mention how a value for the temperature of the mixture during the reaction is obtained.
(i) State the temperature measurements that should be taken and at which stage in the procedure they should be taken.
…… [1]
(ii) State how to use the temperature measurements to determine an accurate value for the temperature of the mixture during the reaction.
…… [1]
(c) A student carries out the procedure at three different temperatures and records the measurements in Table 2.1.
Complete Table 2.1. Record values for temperature to the nearest whole number and the
1 values for to four decimal places. t
Table 2.1
temperature, T / °C time, t / s temperature, T / K 1 –1
/s t
15 176
24 92
32 62
[2]
(d) A second student carries out the procedure at six different temperatures and analyses their data to give the results in Table 2.2.
Table 2.2
log c m
1 –1 1
/K
T t
0.00353 –2.43
0.00336 –1.99
0.00325 –1.68
0.00314 –1.47
0.00302 –1.21
0.00287 –0.82
(i) Use the results from Table 2.2 to plot a graph on the grid in Fig. 2.1 to show the relationship between log c m and . Use a cross (×) to plot each data point. Draw a line
1 1 t T of best fit.
– 0.8
– 1.0
– 1.2
– 1.4
– 1.6
log ( 1t (
– 1.8
– 2.0
– 2.2
– 2.4
– 2.6
0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036
1 –1
/K
T
Fig. 2.1 [2]
(ii) Determine the gradient of your line of best fit in Fig. 2.1. State the coordinates of both points you use in your calculation. These must be selected from your line of best fit. Give the gradient to three significant figures.
coordinates 1 …… coordinates 2 ……
gradient = …… K
[2]
(iii) An equation relating time and temperature variables is shown.
0.434E A log c m = –
1
+ constant t RT
Determine the activation energy, EA, of this reaction using this equation and your answer to (d)(ii).
(If you were unable to find the gradient in (d)(ii), then use the value –3.21 × 103 K. This is not the correct answer.)
Include units in your answer.
Show your working.
EA = ……
units = ……
[3]
(iv) Use your graph to state whether the results from the experiment are reliable. Justify your answer.
…… [1]
(e) Suggest a change to one controlled variable that the student could make so that the time measured for a given temperature is shorter.
…… [1]
[Total: 14]
Important values, constants and standards
molar gas constant R = 8.31 J K–1 mol–1
Faraday constant F = 9.65 × 104 C mol–1
Avogadro constant L = 6.022 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at s.t.p. (101 kPa and 273 K)
Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1)
The Periodic Table of Elements
Group
1 2 13 14 15 16 17 18
1 2
H He hydrogen helium
Key 1.0 4.0
3 4 atomic number 5 6 7 8 9 10
Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon
6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2
11 12 13 14 15 16 17 18
Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon
23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
at www.cambridgeinternational.org after the live examination series.
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton
39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
12
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon
85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3
55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids
Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon
132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – –
87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson
– – – – – – – – – – – – – – – – –
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium
138.9 140.1 140.9 144.2 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium
– 232.0 231.0 238.0 – – – – – – – – – – –
Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download
To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge
Mark scheme
2(a) To ensure the solution(s) (in flasks A and B) are at the same temperature (of the water bath) (before mixing) 1
2(b)(i) (take the temperature of reaction mixture in flask A) 1
1 at the start of reaction / step 7 OR (immediately) before step 7 OR step 6 / (immediately) before the reaction OR
(immediately) after step 5
AND
2 at the end of reaction / step 8 / when cross is no longer visible / (immediately) after step 8
2(b)(ii) calculate the mean (temperature during the reaction) 1
2(c) 2
temperature, T / °C time, t / s temperature, T / K 1 / t / s–1
15 176 288 0.0057
24 92 297 0.0109
39 62 305 0.0161
M1 column 3 correct M2 column 4 correct
2(d)(i) M1 all points plotted correctly 2
M2 straight line of best fit line drawn passing close to all points (except marked anomaly)
2(d)(ii) M1 two acceptable coordinates from line if best fit expressed in the form (x,y) 2
M2 gradient correctly calculated from points listed for M1
answer correctly rounded to three significant figures
2(d)(iii) M1 any correct version of the equation: gradient = – 0.434 x EA / R 3
M2 evaluation of expression to give value for EA
M3 units = kJ mol–1
2(d)(iv) yes, there are no / few anomalous points / most points lie on or near the line (of best fit) 1
OR no, there is an anomalous point(s) / (some) points don’t lie on or near the line (of best fit)
2(e) increase the concentration of one or both of the reactants 1
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Chemistry 9701/53 for May/June 2024 is out of 30 marks, with 1 hour 15 minutes to answer it, across 2 questions.
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