9701 Chemistry November 2025 Question Paper 35

1 Hydrated ethanedioic acid is a diprotic acid with the formula (COOH)2•xH2O where x is an integer.

Ethanedioic acid reacts with manganate(VII) ions when heated. 5(COOH)2(aq) + 2MnO4 –(aq) + 6H+(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l)

You will determine the value of x in (COOH)2•xH2O by titrating a solution containing ethanedioic acid with manganate(VII) ions.

● FA 1 is 6.20 g dm–3 aqueous ethanedioic acid, (COOH)2•xH2O.

● FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4.

● FA 3 is 1.0 mol dm–3 sulfuric acid, H2SO4.

(a) Method

● Fill the burette with FA 2.

● Pipette 25.0 cm3 of FA 1 into a conical flask.

● Use the measuring cylinder to add approximately 20 cm3 of FA 3 to the conical flask.

● Place the conical flask on a tripod and gauze and heat carefully until the temperature of the solution is approximately 70 °C.

● Remove the flame.

● Carefully lift the hot conical flask and place it on the white tile under the burette.

● Add FA 2 drop-wise for the first 2–3 cm3. Any initial pink colouring may take several seconds to disappear.

● If the reaction mixture turns brown, reheat it to about 70 °C. If the brown colour disappears, continue the titration. If the brown colour remains, discard the contents of the flask and begin a new titration.

● The end-point is reached when a permanent pale pink colour is formed.

● Perform a rough titration with FA 2. Record your burette readings in the space below. The rough titre is cm3.

● Carry out as many titrations as you think necessary to obtain consistent results.

● Make sure any recorded results show the precision of your practical work.

● Record all your burette readings and the volume of FA 2 added in each accurate titration.

Results

[7]

(b) From your accurate titration results, calculate a suitable mean value to be used in your calculations.

Show clearly how you obtained this value.

25.0 cm3 of FA 1 required cm3 of FA 2. [1] I II III IV V VI VII , ,

(c) Calculations

(i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to the appropriate number of significant figures. [1]

(ii) Calculate the amount, in mol, of manganate(VII) ions, MnO4 –, in the volume of FA 2 calculated in (b).

amount of MnO4 – = mol [1]

(iii) Calculate the amount, in mol, of ethanedioic acid that reacts with the manganate(VII) ions in (c)(ii).

amount of (COOH)2 = mol

Hence calculate the concentration, in mol dm–3, of ethanedioic acid in FA 1.

concentration of (COOH)2 = mol dm–3

[2]

(iv) Calculate the relative molecular mass, Mr, of the ethanedioic acid in FA 1.

Mr = [1]

(v) Calculate the value of x in (COOH)2•xH2O.

Show your working.

x = [1]

(d) Explain why it is necessary to add FA 3 in each titration [1]

[Total: 15] , ,

2 Hydrated zinc sulfate has the formula ZnSO4•yH2O where y is an integer.

Hydrated zinc sulfate decomposes when heated, losing only its water of crystallisation and becoming anhydrous.

You will determine the value of y in ZnSO4•yH2O by heating the hydrated salt until it becomes anhydrous.

FA 4 is hydrated zinc sulfate, ZnSO4•yH2O.

(a) Method

● Weigh the crucible with its lid. Record the mass in the space for Results.

● Add between 3.20 g and 3.40 g of FA 4 to the crucible.

● Weigh the crucible with its lid and FA 4. Record the mass.

● Place the crucible on the pipeclay triangle. Gently heat the crucible and contents for approximately 2 minutes with the lid on.

● Remove the lid. Heat the crucible and contents strongly for approximately 4 minutes.

● Replace the lid and leave the crucible and residue to cool for at least 5 minutes. You may wish to begin work on Question 3 while the crucible is cooling.

● Weigh the crucible with its lid and its contents. Record the mass.

● Remove the lid. Heat the crucible strongly for approximately 3 minutes.

● Replace the lid and leave the crucible and residue to cool for at least 5 minutes.

● Weigh the crucible with its lid and its contents. Record the mass.

● Calculate the mass of FA 4 used and the mass of residue obtained. Record the masses.

Results

[5] I II III IV V , ,

(b) Calculations

(i) Calculate the amount, in mol, of anhydrous zinc sulfate residue formed in the decomposition of FA 4.

amount of ZnSO4 = mol

Calculate the amount, in mol, of water of crystallisation lost.

amount of H2O = mol

[2]

(ii) Calculate the value of y in the formula ZnSO4•yH2O.

Show your working.

y = [2]

(c) A student suggests using this thermal decomposition method to investigate the number of moles of water of crystallisation in hydrated ethanedioic acid. The teacher says that this method is unsuitable.

Suggest why this method is unsuitable [1]

[Total: 10] , , 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. , ,

3 (a) A bottle labelled FA 5 is thought to contain hydrated zinc sulfate. It would therefore contain zinc ions and sulfate ions as well as water of crystallisation.

(i) Devise and carry out tests to investigate whether zinc ions, sulfate ions and water of crystallisation are present.

Record the tests you carry out and the observations you see in the space provided.

[5]

(ii) Use your observations in (a)(i) to complete Table 3.1 to show whether each species is present in FA 5.

Use a tick (✓) if the species is present.

Use a cross (✗) if the species is not present. Table 3.1 Zn2+ SO4 2– H2O

[1]

(b) You are provided with solid FA 6.

(i) Heat a few crystals of FA 6 in a hard-glass test-tube until no further gas is evolved. Record all your observations.

Leave the test-tube until it is cool. Keep the cooled residue for use in (b)(ii) [2]

(ii) To the cooled residue from (b)(i), add approximately 3 cm depth of distilled water and stir. Filter the solution formed into a test-tube.

The colour of the solution is [1] , ,

(c) You are provided with aqueous solutions FA 7 and FA 8 and with solid FA 9.

FA 7 is an aqueous solution of FA 6.

FA 7, FA 8 and FA 9 contain compounds which all have one metal that is the same but which may be in different oxidation states.

(i) Carry out the following tests on FA 7, FA 8 and FA 9 and record your observations in Table 3.2. For each test use a 1 cm depth of a solution or a spatula measure of solid. Table 3.2 test observations FA 7 FA 8 FA 9 Test 1 Add hydrogen peroxide. Test 2 Add aqueous sodium hydroxide, then leave to stand. Test 3 Add aqueous iron(II) sulfate.

[4]

(ii) Suggest the identity of the metal in FA 6 / FA 7, FA 8 and FA 9.

The metal is [1]

(iii) Complete Table 3.3 to suggest the oxidation state of the metal in FA 6 / FA 7 and FA 8. Table 3.3 FA 6 / FA 7 FA 8 oxidation state

[1]

[Total: 15] , , Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4 +(aq) no ppt. ammonia produced on warming – barium, Ba2+(aq) faint white ppt. is observed unless [Ba2+(aq)] is very low no ppt. calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very low no ppt. chromium(III), Cr3+(aq) grey-green ppt. soluble in excess giving dark green solution grey-green ppt. insoluble in excess copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on contact with air insoluble in excess green ppt. turning brown on contact with air insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown on contact with air insoluble in excess off-white ppt. rapidly turning brown on contact with air insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO3 2– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3 –(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2 –(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO3 2–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O3 2–(aq) gives off-white / pale yellow ppt. slowly with H+ , , 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint , ,