AS & A Level Chemistry - 9701 Paper 3 2017 Summer Zone 1

In this experiment you will determine the relative formula mass of a copper salt by titration.

A solution of the copper salt reacts with excess acidified potassium iodide, producing iodine. This iodine is then titrated with aqueous sodium thiosulfate, using starch indicator.

FA 1 is an aqueous solution of the copper salt prepared by dissolving 26.0 g of the salt to make 1.00 dm³ of solution.
FA 2 is dilute sulfuric acid, \( H_2SO_4 \).
FA 3 is aqueous potassium iodide, KI.
FA 4 is 0.110 mol dm\(^{-3}\) sodium thiosulfate, \( Na_2S_2O_3 \).
starch indicator

(a) Method

Fill the burette with FA 4.
Pipette 25.0 cm\(^3\) of FA 1 into a conical flask.
Use the measuring cylinder to add approximately 10 cm\(^3\) of FA 2 to the same conical flask.
Use the measuring cylinder to add approximately 20 cm\(^3\) of FA 3 to the mixture in the conical flask. The mixture will now be a brown colour, due to iodine produced in the reaction.
Begin your rough titration by adding FA 4 from the burette until the mixture becomes light brown.
Add 10 drops of starch indicator. The mixture will become darker.
Continue titrating until the mixture becomes an off-white colour. This is the end-point.
Add one drop of starch indicator to check that no traces of dark colour are produced. If the mixture stays off-white, the titration is finished. If some dark colour is produced, because iodine is still present, continue the titration.
Record your burette readings and the rough titre in the space below. The rough titre is ............................. cm\(^3\).
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 FA 4 added in each accurate titration.
Keep FA 3 and starch indicator for use in Question 3. [7]

(b) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.
The iodine produced required ............................. cm\(^3\) of FA 4. [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, \( Na_2S_2O_3 \), in the volume of FA 4 calculated in (b).
moles of \( Na_2S_2O_3 \) = ............................. mol

(ii) Balance the equation for the reaction of iodine with sodium thiosulfate. State symbols are not required.
\[ ......I_2 + ........Na_2S_2O_3 \rightarrow ........Na_2S_4O_6 + ........NaI \]

(iii) Using your answer to (ii), calculate the number of moles of iodine that reacted with the number of moles of \( Na_2S_2O_3 \) calculated in (i).
moles of \( I_2 \) = ............................. mol

(iv) Iodine, \( I_2 \), is produced in the reaction between FA 1 and FA 3. FA 3 is in excess.
\[ 2Cu^{2+}(aq) + 4I^-(aq) \rightarrow 2CuI(s) + I_2(aq) \]
Using your answer to (iii), calculate the number of moles of copper(II) ions in 25.0 cm\(^3\) of FA 1.
moles of \( Cu^{2+} \) ions = ............................. mol

(v) Using your answer to (iv) and the information on page 2, calculate the relative formula mass of the copper compound in FA 1.
\( M_r \) of copper compound = ............................. [4]

Malachite is a basic form of copper carbonate in which copper hydroxide is also present. The accepted chemical formula of malachite is CuCO₃·Cu(OH)₂·H₂O.

When malachite is heated, it decomposes as shown.
$$\text{CuCO}_3 \cdot \text{Cu(OH)}_2 \cdot \text{H}_2\text{O(s)} \rightarrow 2\text{CuO(s)} + \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(g)}$$

In this experiment, you will heat malachite to decompose it and use your results to obtain evidence about the accepted formula of malachite.
FA 5 is malachite, CuCO₃·Cu(OH)₂·H₂O.

(a) Method
Read through the method before starting any practical work.
In the space below prepare a single table for your results of Experiments 1 and 2.

Experiment 1
• Weigh a crucible with its lid and record the mass.
• Add between 2.5g and 3.0g of FA 5 to the crucible. Weigh the crucible with FA 5 and lid and record the mass.
• Place the crucible on the pipe-clay triangle.
• Heat the crucible and contents gently for about two minutes, with the lid on.
• Remove the lid and continue heating gently for about three minutes.
• Replace the lid and leave the crucible and residue to cool for at least five minutes. Then reweigh the crucible and contents with the lid on. Record the mass.
• While the crucible is cooling, you may wish to begin work on Question 3.
• Calculate and record the mass of FA 5 used and the mass of residue obtained.
• State the observation(s) you made while the reaction was taking place.

Experiment 2
Repeat the method used in Experiment 1, using between 1.5g and 2.0g of FA 5 in the second crucible.
Results

(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your results from Experiment 1 to calculate the number of moles of copper oxide, CuO, obtained as residue.
Use the Periodic Table on page 12 for any data you may require.
\[ \text{moles of CuO obtained in Experiment 1} = \text{.................... mol} \]
(ii) Use your answer to (i), the equation on page 4 and the mass of FA 5 you used in Experiment 1, to calculate the relative formula mass, $M_r$, of malachite.
\[ M_r \text{ of malachite (from Experiment 1)} = \text{....................} \]
(iii) Use your results from Experiment 2 to calculate another value for the relative formula mass, $M_r$, of malachite.
\[ M_r \text{ of malachite (from Experiment 2)} = \text{....................} \]
(iv) Use data from the Periodic Table to calculate the relative formula mass, $M_r$, of malachite from its accepted formula, CuCO₃·Cu(OH)₂·H₂O.
\[ M_r \text{ of malachite (from formula)} = \text{....................} \]
(v) If the relative formula mass of malachite obtained from either of your experiments is within 2.5% of the answer in (iv), this is good evidence that the accepted formula, CuCO₃·Cu(OH)₂·H₂O, is correct.
Show by calculation whether either of your experiments supports the accepted formula.

(c) (i) State one way of improving the accuracy of the experimental method, using the same masses of FA 5.
Explain the benefit of your improvement.
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(ii) Explain why you would expect Experiment 1 to be more accurate than Experiment 2.
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Qualitative Analysis

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 reagents are selected for use in a test, the name or correct formula of the element or compound must be given.

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.
No additional tests for ions present should be attempted.

If any solution is warmed, a boiling tube MUST be used.
Rinse and reuse test-tubes and boiling tubes where possible.

(a) FA 6 is another salt of copper. The anion present is one of those listed in the Qualitative Analysis Notes.

(i) Transfer a small spatula measure of FA 6 into a hard-glass test-tube.
Heat gently at first, then heat strongly, until no further change occurs.

Record all your observations below.
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(ii) Suggest the chemical formula of FA 6.
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(b) (i) Dissolve the remainder of FA 6 in an approximately 10cm depth of distilled water in a boiling tube.

FA 7 is a solution of a salt containing one anion from those listed in the Qualitative Analysis Notes.
Two cations are also present.

Carry out the tests described below using separate portions of solutions FA 6 and FA 7.
Record your observations in the table.

[Table_1]

(ii) What can you deduce about solution FA 7 from its reaction with magnesium?
Explain your answer.
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(iii) Give the ionic equation for the reaction of the metal cation in FA 7 with aqueous sodium hydroxide. Include state symbols.
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(iv) What type of reaction took place when aqueous potassium iodide was added to FA 7?
Use your observations to help you explain your answer.
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(v) The observation you made when aqueous silver nitrate was added to FA 7 does not allow the anion in FA 7 to be identified with certainty.

Explain why you cannot be certain about the identity of the anion.
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(vi) A student suggested that the anion in FA 7 could be identified with more certainty if excess ammonia solution was added after the aqueous silver nitrate.

Explain why this suggestion is not correct.
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