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The formula of hydrated copper(II) sulfate is $\text{CuSO}_4. x\text{H}_2\text{O}$ where $x$ is the number of moles of water of crystallisation in one mole of salt. You will determine the value of $x$ by titration.
When aqueous copper(II) ions react with aqueous iodide ions, $\text{I}^-$, iodine is produced.
$$2\text{Cu}^{2+}(aq) + 4\text{I}^-(aq) \rightarrow \text{I}_2(aq) + 2\text{CuI}(s)$$
The amount of iodine, $\text{I}_2$, produced can be found by titrating it with aqueous thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$.
$$2\text{S}_2\text{O}_3^{2-}(aq) + \text{I}_2(aq) \rightarrow \text{S}_4\text{O}_6^{2-}(aq) + 2\text{I}^-(aq)$$
FA 1 is aqueous CuSO$_4$.xH$_2$O containing 26.2 g dm$^{-3}$.
FA 2 is 0.100 mol dm$^{-3}$ sodium thiosulfate, Na$_2$S$_2$O$_3$.
FA 3 is aqueous potassium iodide, KI.
Starch indicator
(a) Method
• Pipette 25.0 cm$^3$ of FA 1 into a conical flask.
• Use the measuring cylinder to add 15 cm$^3$ of FA 3, an excess of KI, to the conical flask. The solution will turn brown because iodine is formed.
• Fill the burette with FA 2.
• Add FA 2 from the burette until the colour of the mixture changes to pale brown.
• Add 10 drops of starch indicator. The mixture will turn blue-black.
• Continue adding FA 2 from the burette until the dark colour suddenly disappears to leave an off-white solid. This is the end point of the titration.
• Carry out a rough titration and record your burette readings 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 certain any recorded results show the precision of your practical work.
• Record in a suitable form below, all your burette readings and the volume of FA 2 added in each accurate titration. [7]
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0 cm$^3$ of FA 1 required ............................. cm$^3$ of FA 2. [1]
(c) Calculations
Show your working and appropriate significant figures in each step of your calculations.
(i) Calculate the number of moles of thiosulfate ions present in the volume of FA 2 you have calculated in (b).
moles of S$_2$O$_3^{2-}$ = ............................... mol
(ii) Use your answer to (i), and the equations for the reactions involved, to deduce the number of moles of Cu$^{2+}$ present in 25.0 cm$^3$ of FA 1.
$$2\text{Cu}^{2+}(aq) + 4\text{I}^-(aq) \rightarrow \text{I}_2(aq) + 2\text{CuI}(s)$$
$$2\text{S}_2\text{O}_3^{2-}(aq) + \text{I}_2(aq) \rightarrow \text{S}_4\text{O}_6^{2-}(aq) + 2\text{I}^-(aq)$$
moles of Cu$^{2+}$ = ............................. mol
(iii) Use your answer to (ii) and the mass of CuSO$_4$⋅xH$_2$O present in the solution, to calculate the relative molecular mass, $M_r$, of CuSO$_4$⋅xH$_2$O.
$M_r$ of CuSO$_4$⋅xH$_2$O = .............................
(iv) Determine the value of $x$.
(Use data from the Periodic Table on page 12.)
$x = ...........................$[6]
570
553
521
FA 4 is an impure sample of hydrated calcium chloride, $\text{CaCl}_2.2\text{H}_2\text{O}$. On heating, hydrated calcium chloride loses its water of crystallisation.
\( \text{CaCl}_2.2\text{H}_2\text{O(s)} \rightarrow \text{CaCl}_2\text{(s)} + 2\text{H}_2\text{O(g)} \)
You will determine the purity of FA 4 by measuring the loss in mass that occurs when it is heated. The impurity present in FA 4 is not decomposed on heating.
(a) Method
You should read the instructions carefully before starting any practical work and draw a table for your results in the space below.
● Weigh a crucible and record its mass.
● Add between 1.80 g and 2.00 g of FA 4 into the crucible.
● Reweigh the crucible and its contents and record the mass.
● Place the crucible on the pipe-clay triangle and heat gently for 1 minute and then strongly for a further 2 minutes.
● Allow the crucible and its contents to cool. Reweigh the crucible and contents and record the mass.
● Heat the crucible strongly for a further 2 minutes. Allow it to cool. Reweigh the crucible and contents and record the mass.
● Repeat the heating, cooling and weighing until you are satisfied that all the water of crystallisation has been removed.
● Calculate and record the mass of FA 4 used and the total mass of water lost.
While you are waiting for the crucible to cool, you may wish to start work on Question 3.
[6]
(b) Calculations
Show your working and appropriate significant figures in the final answer to each stage of your calculations.
(i) The percentage loss in mass on heating is defined as
\( \frac{\text{the loss in mass on heating}}{\text{the original mass}} \times 100. \)
Calculate the percentage loss in mass of FA 4.
percentage loss in mass = .......................... %
(ii) Calculate the percentage loss in mass when pure hydrated calcium chloride, $\text{CaCl}_2.2\text{H}_2\text{O}$, is heated.
percentage loss in mass = .......................... %
(iii) Use your results to (i) and (ii) to calculate the percentage purity of FA 4, impure $\text{CaCl}_2.2\text{H}_2\text{O}$.
percentage purity = .......................... %
[3]
(c) A student carried out this experiment using 2.60 g of FA 4.
Suggest whether this experiment would give a more accurate result for the percentage purity of FA 4. Explain your answer.
.................................................................................................................................................. .................................................................................................................................................. [1]
(d) In your calculations you assumed that the impurity in FA 4 does not decompose on heating.
State how the percentage purity that you calculated in (b)(iii) would change if the impurity were to decompose on heating. Explain your answer.
.................................................................................................................................................. .................................................................................................................................................. [1]
507
511
559
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 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.
Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.
(a) (i) FA 5, FA 6 and FA 7 are aqueous solutions each containing one anion and one cation.
Carry out the experiments described below and record your observations for each solution in the table.
| | FA 5 | FA 6 | FA 7 |
|---|---|---|---|
| To a 1 cm depth in a test-tube, add a 1 cm depth of aqueous sodium carbonate. | | | |
| To a 1 cm depth in a test-tube, add a 1 cm depth of aqueous copper(II) sulfate. | | | |
| To a 1 cm depth in a test-tube, add a 1 cm depth of aqueous barium chloride or aqueous barium nitrate. | | | |
(ii) What ion is present in both FA 6 and FA 7?
..........................................................................................................................................................
(iii) The anion in FA 5 is one of carbonate, chloride, nitrate or sulfate. Which anion is present in FA 5?
..........................................................................................................................................................
(iv) Write the ionic equation, including state symbols, for the reaction between FA 5 and aqueous copper(II) sulfate.
.........................................................................................................................................................
(b) FA 8 contains two anions and two cations from the lists on pages 10 and 11.
• To a 5 cm depth of distilled water in a boiling tube, add all the FA 8.
• Shake the boiling tube thoroughly for one minute to make sure that no more of the solid will dissolve.
• Filter the mixture into a clean boiling tube.
• Place the filter funnel in a conical flask and wash the residue with a little distilled water.
• Keep both filtrate and residue for tests (i) and (ii) below.
(i) Tests on the filtrate (the solution in the boiling tube)
Carry out the following tests and record your observations in the table below.
| test | observations |
|---|---|
| To a 1 cm depth of the filtrate in a test-tube, add aqueous sodium hydroxide, then | |
| add aqueous hydrogen peroxide. | |
(ii) Tests on the residue
Carry out the following tests and record your observations in the table below.
| test | observations |
|---|---|
| Place the funnel containing the residue into a clean boiling tube. Pour approximately 5 cm³ of dilute nitric acid onto the residue. Collect a 1 cm depth of solution in the boiling tube. Remove the funnel and return it to the conical flask. | |
| To this solution in the boiling tube, add aqueous sodium hydroxide. | |
(iii) Identify two cations present in FA 8.
cations present .......................... and ............................
(iv) Identify one anion present in FA 8.
anion present .........................
(v) Suggest what type of reaction is happening when hydrogen peroxide is added in test (b)(i).
.........................................................................................................................................................
524
598
528
