AS & A Level Chemistry - 9701 Paper 3 2010 Winter Zone 4

FB 1 is an aqueous solution containing 21.50 g dm^-3 of a mixture of iron(II) sulfate, FeSO₄ and iron(III) sulfate, Fe₂(SO₄)₃.
FB 2 is an aqueous solution containing 2.00 g dm^-3 potassium manganate(VII), KMnO₄.

In the presence of acid, the iron(II) sulfate is oxidised by potassium manganate(VII).

2KMnO₄(aq) + 8H₂SO₄(aq) + 10FeSO₄(aq) → 5Fe₂(SO₄)₃(aq) + 2MnSO₄(aq) + K₂SO₄(aq) + 8H₂O(l)

(a) Method

- Fill a burette with FB 2.
- Pipette 25.0 cm³ of FB 1 into the conical flask.
- Use a 25 cm³ measuring cylinder to add 10 cm³ of dilute sulfuric acid to the flask.
- Place the flask on a white tile.
- Carefully titrate with FB 2 until the first permanent pink colour is obtained.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

The rough titre is ......................... cm³.

- Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Record in a suitable form below all of your burette readings and the volume of FB 2 added in each accurate titration.
- Make certain any recorded results show the precision of your practical work.

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

25.0 cm³ of FB 1 required ............. cm³ of FB 2.

(c) Calculations

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

(i) Calculate the concentration, in mol dm^-3, of the potassium manganate(VII) in FB 2.
FB 2 contains 2.00 g dm^-3 KMnO₄.
[A_r: O, 16.0; K, 39.1; Mn, 54.9]

The concentration of potassium manganate(VII) in FB 2 is .......................... mol dm^-3.

(ii) Calculate how many moles of KMnO₄ were present in the volume calculated in (b).

.......................... mol of KMnO₄.

(iii) Calculate how many moles of iron(II) sulfate, FeSO₄, reacted with the potassium manganate(VII) in (ii).
2KMnO₄(aq) + 8H₂SO₄(aq) + 10FeSO₄(aq) → 5Fe₂(SO₄)₃(aq) + 2MnSO₄(aq) + K₂SO₄(aq) + 8H₂O(l)

.......................... mol of FeSO₄ reacted with the potassium manganate(VII).

(iv) Calculate the concentration, in mol dm^-3 of FeSO₄ in FB 1.

The concentration of FeSO₄ in FB 1 is ......................... mol dm^-3.

(v) Calculate the concentration, in g dm^-3, of FeSO₄ in FB 1.
[A_r: O, 16.0; S, 32.1; Fe, 55.8]

FB 1 contains .......................... g dm^-3 of FeSO₄.

(vi) FB 1 is an aqueous solution containing 21.50 g dm^-3 of FeSO₄ and Fe₂(SO₄)₃.
Calculate the percentage, by mass, of FeSO₄ in this mixture.

The mixture contains .......................... % FeSO₄.

FB 3 is a mixture containing anhydrous sodium carbonate, Na\textsubscript{2}CO\textsubscript{3}, and sodium hydrogencarbonate, NaHCO\textsubscript{3}.
When heated, sodium hydrogencarbonate decomposes.
2NaHCO\textsubscript{3}(s) \rightarrow Na\textsubscript{2}CO\textsubscript{3}(s) + CO\textsubscript{2}(g) + H\textsubscript{2}O(g)
Anhydrous sodium carbonate does not decompose when heated.
You are to determine if sodium hydrogencarbonate is the major component, by mass, of the mixture in FB 3.
(a) Method – Read through the instructions before starting any practical work.
• Weigh and record the mass of an empty boiling-tube.
• Tip the contents of the tube labelled FB 3 into the weighed boiling-tube. Reweigh and record the mass of the boiling-tube and FB 3.
• Gently heat the FB 3 in the boiling-tube for 2 minutes then heat strongly for a further 2 minutes.
Take care not to lose any solid from the tube during heating.
• Warm the upper parts of the boiling-tube to evaporate any water that may have condensed while heating the solid.
• Place the hot tube on a heat-proof mat and leave to cool.
You are advised to continue with part (d) of this question or to start another question while the tube cools.
• When cool, reweigh the boiling-tube and the residual sodium carbonate.
• Reheat, cool and reweigh the tube until you are satisfied decomposition is complete.

Results
In an appropriate form, in the space below, record all of your balance readings, the mass of FB 3 heated, the mass of residual sodium carbonate and the mass loss on heating.

(b) Do not use your experimental results in part (i)
(i) Use the equation for the decomposition of NaHCO\textsubscript{3} on heating to calculate the theoretical ratio $\frac{\text{mass of NaHCO\textsubscript{3}}}{\text{mass loss on heating}}$
2NaHCO\textsubscript{3}(s) \rightarrow Na\textsubscript{2}CO\textsubscript{3}(s) + CO\textsubscript{2}(g) + H\textsubscript{2}O(g)
[Mr: NaHCO\textsubscript{3}, 84.0; CO\textsubscript{2}, 44.0; H\textsubscript{2}O, 18.0]

theoretical ratio = ..................................................

(ii) Use the following expression to calculate the mass of NaHCO\textsubscript{3} in the sample of FB 3 that was heated.

theoretical ratio from b(i) $\times$ experimental mass loss from (a)

mass of NaHCO\textsubscript{3} = ............................ g

(iii) Tick the appropriate box in the table below.

FB 4, FB 5, FB 6 and FB 7 are aqueous solutions each containing one of the ions Al^{3+}, NH_4^+, Mg^{2+}, Mn^{2+}.

You will carry out the following tests on each of the solutions.

At each stage of any test you are to record details of the following.

- colour changes seen
- the formation of any precipitate
- the solubility of such precipitates in an excess of the reagent added

Where gases are released they should be identified by a test, described in the appropriate place in your observations.

You should indicate clearly at what stage in a test a change occurs. Marks are not given for chemical equations. No additional tests for ions present should be attempted. If any solution is warmed, a boiling-tube MUST be used.

(a) Carry out the following tests. Record your observations in the spaces provided in the table.

[Table_1]

(i) To 1 cm depth of solution in a test-tube add 1 cm depth of aqueous sodium hydroxide. Swirl the tube, then add a further 2 cm depth of aqueous sodium hydroxide.

In tests (ii) and (iii) put a cross in any boxes where the test is not carried out.

(ii) If a precipitate remains at the end of test (i) leave the test-tube and contents to stand for a few minutes.

(iii) If no precipitate formed at all in test (i) tip the contents of the tube into a boiling-tube and warm gently.
Care: heated solutions containing sodium hydroxide are liable to be ejected from the tube.

(b) Use the Qualitative Analysis Notes on page 13 to identify the cation present in each of the solutions. Complete the table below to identify each ion and to give supporting evidence from your observations.

[Table_2]

Rinse and re-use test-tubes where possible.

(c) Carry out the following tests on the solution you have identified as containing Al^{3+} ions and record your observations in the spaces provided.

[Table_3]
(i) Add aqueous sodium iodide
(ii) Add dilute sulfuric acid

Explain how your results confirm the presence of Al^{3+} and eliminate any other ion.

(d) What other cation listed in the Qualitative Analysis Notes on page 13 would give similar results to Al^{3+} in (a)?

(e) Carry out the following tests and make careful observations of all that happens in each experiment. Complete the table.

[Table_4]
(i) To 1 cm depth of aqueous silver nitrate in a test-tube add 1 cm depth of aqueous sodium chloride. Keep the tube for comparison with the observations in test (ii).

(ii) Repeat test (i). To 1 cm depth of aqueous silver nitrate in a test-tube add 1 cm depth of aqueous sodium chloride, then add 1 cm depth of aqueous sodium iodide and shake the tube.

(f) Suggest an explanation for your observations when aqueous sodium iodide is added in test (e)(ii).