AS & A Level Chemistry - 9701 Paper 3 2013 Winter Zone 6

In this experiment you are to determine the concentration of aqueous potassium manganate(VII), FB 3, by titration.

In the titration potassium manganate(VII) is first reacted with acidified potassium iodide to produce iodine. The amount of iodine formed is then determined by titrating the mixture with sodium thiosulfate.

FB 1 is hydrated sodium thiosulfate, Na₂S₂O₃·5H₂O.
FB 2 is dilute sulfuric acid, H₂SO₄.
FB 3 is aqueous potassium manganate(VII), KMnO₄.
FB 4 is aqueous potassium iodide, KI.
starch indicator

(a) Method

Preparing a solution of FB 1

• Weigh the 250 cm³ beaker and record the mass in the space below.
• Add all the FB 1 to the beaker. Weigh the beaker with FB 1 and record the mass.
• Calculate the mass of FB 1 used and record this in the space below.
• Add approximately 100 cm³ of distilled water to the beaker. Stir until all the solid has dissolved.
• Transfer the solution into the 250 cm³ volumetric (graduated) flask labelled FB 5.
• Wash out the beaker thoroughly using distilled water and add the washings to the volumetric flask. Make the solution up to the mark using distilled water.
• Shake the flask thoroughly to mix the solution before using it for your titrations.
• This solution of sodium thiosulfate is FB 5.

Titration

• Use the measuring cylinder to add 20 cm³ of FB 2 to a conical flask.
• Use the measuring cylinder to add 10 cm³ of FB 4 to the same flask.
• Pipette 25.0 cm³ of FB 3 into the same flask.
The colour of the mixture is caused by iodine.
• Fill the burette with FB 5.
• Begin each titration without adding the starch indicator.
Add 10 drops of starch indicator when the colour of the mixture becomes (pale) yellow.
The end-point is when the blue-black colour caused by the starch disappears.
• 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 below all of your burette readings and the volume of FB 5 added in each accurate titration.

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(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

suitable value = .................. cm³ of FB 5 [1]

(c) Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate the number of moles of sodium thiosulfate, FB 1, that were weighed out. The relative formula mass of hydrated sodium thiosulfate is 248.2.

moles of sodium thiosulfate = ....................... mol

(ii) Calculate the number of moles of sodium thiosulfate that were present in the volume of FB 5 calculated in (b).

moles of sodium thiosulfate = ....................... mol

(iii) Iodine produced by the reaction in the conical flask reacts with sodium thiosulfate.
Use the equation below to calculate the number of moles of iodine that reacted with sodium thiosulfate in (ii).

I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆

moles of I₂ = ....................... mol

(iv) The iodine is produced as a result of the oxidation of iodide ions in potassium iodide, FB 4, by potassium manganate(VII), FB 3.
The ionic equation for this reaction is

2MnO₄⁻ + 16H⁺ + 10I⁻ → 2Mn²⁺ + 5I₂ + 8H₂O

Calculate the number of moles of potassium manganate(VII), KMnO₄, that reacted to produce the iodine in (iii).

moles of KMnO₄ = ...................... mol

(v) Calculate the concentration of potassium manganate(VII), in g dm⁻³, in FB 3.
($A_r$: O, 16.0 ; K, 39.1; Mn, 54.9)

concentration of KMnO₄ = ...................... g dm⁻³ [5]

(d) (i) State the maximum error in any single reading of the burette.

maximum error = ..................... cm³

(ii) Calculate the maximum percentage error in volume of FB 5 in your first accurate titre.

maximum percentage error = ...................... % [1]

In this experiment you will heat two separate samples of a hydrated salt to drive off the water of crystallisation. You will then calculate the relative atomic mass of the metal in the salt.
FB 6 is the hydrated salt. The formula of FB 6 is MSO$_4$·7H$_2$O, where M is the metal.
(a) Method
Record all weighings, in an appropriate form, in the space below.
• Record the mass of the empty crucible without its lid.
• Add between 2.0 and 2.4 g of FB 6 into the crucible. Record the mass of the crucible and its contents.
• Use a pipe-clay triangle to support the crucible and contents on a tripod.
• Heat the crucible and its contents gently and carefully for about two minutes, with the lid off. Then heat very strongly for a further three minutes.
• Put the lid on the crucible and leave it to cool for approximately 10 minutes.

While you are waiting for the crucible to cool, start work on Question 3.
• When the crucible is cool, remove the lid, and weigh the crucible with the residue.
• Record the mass of anhydrous MSO$_4$ remaining in the crucible after heating and therefore calculate the mass of water lost.
• To prepare for the second experiment, use a spatula to remove the solid residue from the crucible into the beaker labelled waste.
• Reweigh the empty crucible without its lid.
• Carry out the experiment again. This time use between 2.5 and 2.9 g of FB 6.

(b) Calculation
Show your working and give your answers to three significant figures.

(i) State and explain which of your two experiments in (a) is likely to be the more accurate.
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(ii) Calculate the number of moles of water removed from the hydrated salt in the more accurate experiment.
($A_r$: H, 1.0; O, 16.0)

moles of H$_2$O = .................. mol

(iii) Complete the equation for the removal of water from hydrated FB 6. Include state symbols.
MSO$_4$·7H$_2$O (.....) $\rightarrow$ ............(.....) + ............(.....)

(iv) Using your answer to (ii), calculate the number of moles of anhydrous MSO$_4$ produced in the more accurate experiment.

moles of MSO$_4$ = ............. mol

(v) Use the mass of anhydrous MSO$_4$ produced in the more accurate experiment to calculate the relative formula mass of MSO$_4$.

relative formula mass of MSO$_4$ = ..................

(vi) Calculate the relative atomic mass of M.
($A_r$: O, 16.0; S, 32.1)
(If you were unable to calculate the relative formula mass of anhydrous MSO$_4$ you may assume that it was 126.3. This is not the correct value.)

$A_r$ of M = .............

(vii) The relative atomic masses of some of the cations on page 11 are given below.
($A_r$: Mg, 24.3; Ca, 40.0; Fe, 55.8; Cu, 63.5; Mn, 54.9; Zn, 65.4)
M is a cation of one of the elements listed above. Suggest the identity of M and justify your answer.
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(viii) Suggest why it was not necessary to include the cations aluminium and chromium from page 11 in the list of relative atomic masses in (vii).
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(c) The crucible was cooled with the lid on to prevent absorption of water vapour from the air. Suggest a better way of preventing water vapour being absorbed during cooling.
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(a) You will carry out further tests on the ions in FB 6.

- Put a spatula measure of FB 6 into a test-tube.
- Half fill the test-tube with distilled water and stir until the solid dissolves.
- Use a 1 cm depth of the solution of FB 6 in separate test-tubes for the tests you will carry out.

(i) Add aqueous sodium hydroxide to FB 6 solution.
Add aqueous ammonia to FB 6 solution.
Record your observations below.

(ii) Carry out a test of your choice to show that sulfate ions are present in FB 6.

reagent(s) used ...........................................................

observation(s) ..................................................................
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(iii) Give the ionic equation for the reaction in test (ii).
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(b) FB 7, FB 8 and FB 9 are aqueous solutions, each containing one cation and one anion.
None of the cations and none of the anions in FB 7, FB 8 and FB 9 are identical.

(i) Add a 2 cm magnesium strip to a 2 cm depth of each solution in a clean test-tube.
Mix pairs of solutions as shown so that you can complete the table shown below.
Use 1 cm depths of solutions in clean test-tubes.
Record your observations in the table.

[Table with placeholders]

(ii) The anion present in FB 7 is the sulfate ion.
Identify FB 7, giving evidence from your observations.

FB 7 is ....................................... .
evidence ......................................................................
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(iii) Add a 1 cm depth of aqueous hydrogen peroxide, FB 10, to the mixture of FB 7 and FB 9 that you kept from (i). Then add three drops of starch.
Record your observation(s).
Identify the coloured chemical produced when hydrogen peroxide was added to the mixture of FB 7 and FB 9 and name the anion present in FB 9.

observations ..............................................................
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chemical produced ......................................
anion in FB 9 ...................................................

(iv) Give the chemical formula of the substance you observed when solutions FB 8 and FB 9 were mixed.
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