9701 Chemistry November 2025 Question Paper 52

1 Aqueous silver ions, Ag+(aq), react slowly with aqueous iron(II) ions, Fe2+(aq). An equilibrium is established. Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

The concentration of Ag+(aq) at equilibrium can be determined by titration with a standard solution of aqueous potassium thiocyanate, KSCN(aq).

During the titration, the remaining Ag+(aq) ions react with SCN–(aq) ions to form a precipitate of AgSCN(s). Ag+(aq) + SCN–(aq) AgSCN(s)

When all Ag+(aq) ions have been removed from solution, excess SCN–(aq) ions react with Fe3+(aq) to form a complex ion, FeSCN2+(aq), which has a red colour. Fe3+(aq) + SCN–(aq) FeSCN2+(aq)

The appearance of the red colour indicates the end-point.

A student carries out an experiment to determine the equilibrium constant, Kc. Kc = [Fe3+(aq)]eqm [Fe2+(aq)]eqm [Ag+(aq)]eqm

The student makes 250.0 cm3 of 0.0200 mol dm–3 KSCN(aq) to use in the titration.

(a) Calculate the mass of solid potassium thiocyanate, KSCN(s), needed to make 250.0 cm3 of 0.0200 mol dm–3 KSCN(aq).

mass of KSCN(s) = g [1] , ,

(b) Describe how the student should make 250.0 cm3 of 0.0200 mol dm–3 KSCN(aq) starting from the mass of KSCN(s) calculated in (a) in a 50 cm3 beaker.

Give the name and size of any key apparatus used.

Write your answer using a series of numbered steps [3]

(c) The student uses the following method to determine Kc.

step 1 Add 25.0 cm3 of 0.100 mol dm–3 aqueous silver nitrate, AgNO3(aq), into a dry conical flask. Label this flask A.

step 2 Add 25.0 cm3 of 0.100 mol dm–3 aqueous iron(II) sulfate, FeSO4(aq), into flask A.

step 3 Seal flask A, using a bung.

step 4 Allow flask A to stand for twelve hours.

step 5 Transfer 10.0 cm3 of the mixture from flask A into another conical flask, flask B, without disturbing the precipitate in flask A.

step 6 Titrate the sample in flask B with 0.0200 mol dm–3 aqueous potassium thiocyanate, KSCN(aq).

step 7 Repeat steps 5 and 6 until concordant values are obtained.

(i) Suggest why flask A is sealed with a bung in step 3 [1]

(ii) Suggest why flask A is left to stand for twelve hours in step 4 [1]

(iii) Identify the precipitate in flask A in step 5 [1] , ,

(d) The student’s results are shown in Table 1.1

Table 1.1 rough titration titration 1 titration 2 titration 3 final burette reading / cm3 22.50 21.75 31.65 32.20 initial burette reading / cm3 0.00 0.00 9.75 10.20 titre / cm3 22.50 21.75 21.90 22.00

(i) State if concordant titres have been achieved.

Explain your answer [1]

(ii) Calculate the percentage error in the titre volume in titration 2.

Show your working.

percentage error = [1]

(iii) The student repeats the experiment using KSCN(aq) at a higher concentration. The student obtains smaller titres.

Suggest one reason why a larger titre is better than a smaller titre [1] , ,

(e) Another student calculates a mean titre of 21.85 cm3. Use this value to complete the following calculation.

(i) Calculate [Ag+(aq)] in the equilibrium mixture in flask A.

[Ag+(aq)]eqm = mol dm–3 [1]

(ii) Calculate [Fe3+(aq)] in the equilibrium mixture in flask A.

[Fe3+(aq)]eqm = mol dm–3 [1]

(iii) The formula for the equilibrium constant, Kc, is shown. Kc = [Fe3+(aq)]eqm [Fe2+(aq)]eqm [Ag+(aq)]eqm

Determine the value of Kc.

Give the units of Kc.

Kc = units [2] , ,

(f) Several other students perform the same experiment at different temperatures. The Kc values that they obtain are used to produce the graph in Fig. 1.1. 280 2.5 2.7 2.9 Kc / units not shown 3.1 3.3 2.6 2.8 3.0 3.2 3.4 3.5 300 320 340 360 temperature / K 380

Fig. 1.1

(i) One student suggests that Kc is directly proportional to temperature.

State and explain if the results displayed in Fig. 1.1 support this suggestion [1]

(ii) Another student suggests that the data represented in the graph in Fig. 1.1 is reliable.

Explain how the graph supports this suggestion [1] , ,

(iii) Use the data displayed in Fig. 1.1 to state if the forward reaction is exothermic or endothermic. Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

Explain your answer. forward reaction explanation [1] [Total: 17] , ,

2 A student carries out an experiment to determine the concentration of aqueous sulfate ions, SO4 2–(aq), in a sample of lake water.

(a) The student uses the following method.

step 1 Transfer 25.00 cm3 of the lake water sample to a beaker and record its conductivity as shown in Fig. 2.1.

step 2 Add 5.00 cm3 of 0.100 mol dm–3 aqueous barium hydroxide, Ba(OH)2(aq), to the beaker.

step 3 Stir the mixture and record the conductivity of the contents of the beaker as shown in Fig. 2.1.

step 4 Repeat steps 2 and 3 until a total of 40.00 cm3 of 0.100 mol dm–3 Ba(OH)2(aq) has been added to the beaker. 0.0 conductivity meter

Fig. 2.1

(i) Suggest a suitable piece of apparatus to transfer 25.00 cm3 of the lake water sample to the beaker in step 1 [1]

(ii) 0.100 mol dm–3 Ba(OH)2(aq) is an irritant to skin and eyes. Other than wearing safety goggles, state one safety precaution that the student should take when conducting this experiment [1] , ,

(b) The student’s results are given in Table 2.1.

A correction can be applied to the conductivity values to take into account dilution of the solution as its volume increases using the following equation. corrected conductivity = measured conductivity × (total volume in beaker) 25.00

Table 2.1 reading number volume of 0.100 mol dm–3 Ba(OH)2(aq) added to beaker / cm3 total volume in beaker / cm3 measured conductivity / µS cm–1 corrected conductivity / µS cm–1 1 0.00 25.00 37 000 37 000 2 5.00 23 000 27 600 3 10.00 12 000 16 800 4 15.00 2 300 3 680 5 20.00 5 000 6 25.00 12 000 7 30.00 18 000 8 35.00 21 000 9 40.00 24 500

(i) Complete Table 2.1. [2]

(ii) Identify the independent variable in this experiment [1] , ,

(c) Plot a graph on the grid in Fig. 2.2 to show the relationship between corrected conductivity and volume of 0.100 mol dm–3 Ba(OH)2 added to beaker.

Use a cross (×) to plot each data point.

Draw a line of best fit using readings 1 to 4 and another line of best fit using readings 5 to 9. Extend the lines so that they intersect. 0 0 10 20 30 5 15 25 35 40 10 000 20 000 30 000 40 000 50 000 60 000 70 000 rected nductivity S cm–1 volume of 0.100 mol dm–3 Ba(OH)2(aq) added to beaker / cm3

Fig. 2.2

[2] , ,

(d) The point on the graph where the two lines intersect indicates the volume of 0.100 mol dm–3 Ba(OH)2(aq) required to react exactly with the SO4 2–(aq) present in 25.00 cm3 of lake water being tested.

(i) Use the graph in Fig. 2.2 to determine the volume of 0.100 mol dm–3 Ba(OH)2(aq) required to react exactly with SO4 2–(aq) in the lake water sample.

volume required = cm3 [1]

(ii) The equation for the reaction taking place in the beaker is shown. Ba2+(aq) + SO4 2–(aq) BaSO4(s)

Use your answer in (d)(i) to calculate the concentration of SO4 2–(aq) in the lake water sample.

concentration of SO4 2–(aq) = mol dm–3 [1] , ,

(e) The concentration of SO4 2–(aq) in a sample of water can also be determined by measuring the mass of precipitate produced when excess Ba(OH)2(aq) is added to the sample.

The student suggests the following method.

step 1 Place 25.0 cm3 of the water sample in a conical flask.

step 2 Add excess 0.100 mol dm–3 Ba(OH)2(aq) to the flask.

step 3 Filter the contents of the flask.

step 4 Dry the residue in a warm oven.

step 5 Measure the mass of residue.

(i) Draw a labelled diagram to describe the arrangement of apparatus that would be needed to complete step 3.

[1]

(ii) Suggest a step that the student should add between steps 3 and 4 to improve this method [1]

(iii) Describe what the student can do to ensure that the residue weighed in step 5 is completely dry [1]

(iv) The mass of residue is used to calculate the concentration of SO4 2–(aq).

Suggest the effect, if any, on the concentration of SO4 2–(aq) that is calculated if the residue is not completely dried in step 4.

Explain your answer. effect on concentration of SO4 2–(aq) calculated explanation [1] [Total: 13] , ,

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 = 6.02 × 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) , , Group The Periodic Table of Elements 1 H hydrogen 1.0 2 He helium 4.0 1 2 13 14 15 16 17 18 3 4 5 6 7 8 9 10 11 12 3 Li lithium 6.9 4 Be beryllium 9.0 atomic number atomic symbol Key name relative atomic mass 11 Na sodium 23.0 12 Mg magnesium 24.3 19 K potassium 39.1 20 Ca calcium 40.1 37 Rb rubidium 85.5 38 Sr strontium 87.6 55 Cs caesium 132.9 56 Ba barium 137.3 87 Fr francium – 88 Ra radium – 5 B boron 10.8 13 Al aluminium 27.0 31 Ga gallium 69.7 49 In indium 114.8 81 Tl thallium 204.4 6 C carbon 12.0 14 Si silicon 28.1 32 Ge germanium 72.6 50 Sn tin 118.7 82 Pb lead 207.2 22 Ti titanium 47.9 40 Zr zirconium 91.2 72 Hf hafnium 178.5 104 Rf rutherfordium – 23 V vanadium 50.9 41 Nb niobium 92.9 73 Ta tantalum 180.9 105 Db dubnium – 24 Cr chromium 52.0 42 Mo molybdenum 95.9 74 W tungsten 183.8 106 Sg seaborgium – 25 Mn manganese 54.9 43 Tc technetium – 75 Re rhenium 186.2 107 Bh bohrium – 26 Fe iron 55.8 44 Ru ruthenium 101.1 76 Os osmium 190.2 108 Hs hassium – 27 Co cobalt 58.9 45 Rh rhodium 102.9 77 Ir iridium 192.2 109 Mt meitnerium – 28 Ni nickel 58.7 46 Pd palladium 106.4 78 Pt platinum 195.1 110 Ds darmstadtium – 29 Cu copper 63.5 47 Ag silver 107.9 79 Au gold 197.0 111 Rg roentgenium – 30 Zn zinc 65.4 48 Cd cadmium 112.4 80 Hg mercury 200.6 112 Cn copernicium – 114 Fl flerovium – 116 Lv livermorium – 7 N nitrogen 14.0 15 P phosphorus 31.0 33 As arsenic 74.9 51 Sb antimony 121.8 83 Bi bismuth 209.0 8 O oxygen 16.0 16 S sulfur 32.1 34 Se selenium 79.0 52 Te tellurium 127.6 84 Po polonium – 9 F fluorine 19.0 17 Cl chlorine 35.5 35 Br bromine 79.9 53 I iodine 126.9 85 At astatine – 10 Ne neon 20.2 18 Ar argon 39.9 36 Kr krypton 83.8 54 Xe xenon 131.3 86 Rn radon – 113 Nh nihonium – 115 Mc moscovium – 117 Ts tennessine – 118 Og oganesson – 21 Sc scandium 45.0 39 Y yttrium 88.9 57–71 lanthanoids 89–103 actinoids 57 La lanthanum 138.9 89 Ac lanthanoids actinoids actinium – 58 Ce cerium 140.1 90 Th thorium 232.0 59 Pr praseodymium 140.9 91 Pa protactinium 231.0 60 Nd neodymium 144.2 92 U uranium 238.0 61 Pm promethium – 93 Np neptunium – 62 Sm samarium 150.4 94 Pu plutonium – 63 Eu europium 152.0 95 Am americium – 64 Gd gadolinium 157.3 96 Cm curium – 65 Tb terbium 158.9 97 Bk berkelium – 66 Dy dysprosium 162.5 98 Cf californium – 67 Ho holmium 164.9 99 Es einsteinium – 68 Er erbium 167.3 100 Fm fermium – 69 Tm thulium 168.9 101 Md mendelevium – 70 Yb ytterbium 173.1 102 No nobelium – 71 Lu lutetium 175.0 103 Lr lawrencium – , ,