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In this experiment you will determine the value of x in the formula for hydrated copper(II) sulfate, CuSO₄·xH₂O. You will first react a solution of Cu²⁺ ions with excess iodide ions, I⁻. This reaction produces iodine.
2Cu²⁺(aq) + 4I⁻(aq) → 2CuI(s) + I₂(aq)
The amount of iodine produced can be determined by titrating with thiosulfate ions, S₂O₃²⁻.
I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)
FA 1 is 0.150 mol dm ⁻³ sodium thiosulfate, Na₂S₂O₃.
FA 2 is dilute sulfuric acid.
FA 3 is 1.00 mol dm⁻³ potassium iodide, KI.
FA 4 is a solution made by dissolving 32.5 g of CuSO₄ . xH₂O in 1.00 dm³ of solution.
starch indicator
(a) Method
- Fill the burette with FA 1.
- Pipette 25.0 cm³ of FA 4 into a conical flask.
- Use the measuring cylinder to add 10 cm³ of FA 2 to the same conical flask.
- Use the measuring cylinder to add 10 cm³ of FA 3 to the same conical flask. The mixture will become brown because of the formation of I₂, and will become cloudy because of the formation of the white precipitate of CuI.
- Add FA 1 from the burette until the mixture becomes a light brown colour.
- Add 10 to 20 drops of starch indicator until the mixture becomes blue-black.
- Continue to titrate with FA 1 until the blue-black colour disappears leaving a mixture with an off-white solid. This is the end-point.
- You should test that the end-point has been reached by adding 2 more drops of starch indicator. If the titration has reached the end-point the added starch indicator will cause no change in colour.
- 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 FA 1 added in each accurate titration.
[7]
(b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations.
Show clearly how you obtained this value.
25.0 cm³ of FA 4 required .............................. cm³ of FA 1. [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 thiosulfate ions in the volume of FA 1 calculated in (b).
moles of S₂O₃²⁻ = .............................. mol
(ii) Using the equations on page 2, calculate the number of moles of copper(II) ions in 25.0 cm³ of FA 4.
moles of Cu²⁺ = .............................. mol
(iii) Calculate the concentration, in mol dm⁻³, of copper(II) ions in FA 4.
concentration of Cu²⁺ in FA 4 = .............................. mol dm⁻³
(iv) Calculate the value of x in CuSO₄·xH₂O.
x = ..............................
[5]
(d) (i) Calculate the maximum percentage error in one of your accurate titres.
maximum percentage error = .............................. %
(ii) A student suggests that the experiment could be made more accurate if the volume of FA 3 was measured using a burette.
Give a reason why the student might make this suggestion.
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Explain why this change would not improve the accuracy of the experiment.
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[3]
In this experiment you will determine the value of y in the formula for hydrated barium chloride, $BaCl_2·yH_2O$. You will do this by measuring the mass loss when a sample of hydrated barium chloride is heated.
$BaCl_2·yH_2O(s) \rightarrow BaCl_2(s) + yH_2O(g)$
$FA 5$ is hydrated barium chloride, $BaCl_2·yH_2O$.
(a) Method
Before starting any practical work, read through all the instructions and prepare a table for your results in the space provided.
• Weigh the crucible with a lid and record the mass.
• Add all the $FA 5$ to the crucible.
• Reweigh the crucible with the lid and $FA 5$. Record the mass.
• Place the crucible in the pipe-clay triangle on top of a tripod.
• Heat the crucible gently with the lid on for about 1 minute.
• Remove the lid and then heat more strongly for a further 4 minutes.
• Replace the lid and allow the crucible to cool.
• While the crucible is cooling you may wish to begin work on Question 3.
• Once the crucible has cooled, reweigh the crucible with the lid and contents. Record the mass.
• Calculate and record the mass of $FA 5$ used, the mass of the residue and the mass of water lost.
(b) 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 barium chloride in the residue. You may assume all the water has been removed.
moles of $BaCl_2$ = .............................. mol
(ii) Calculate the number of moles of water lost.
moles of $H_2O$ lost = .............................. mol
(iii) Calculate the value of y in $BaCl_2·yH_2O$.
$y$ = ..............................
(c) (i) For this experiment to give an accurate value for y, anhydrous barium chloride must be thermally stable.
Explain fully what would happen to the value of y if $BaCl_2$ were to decompose slightly during heating.
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(ii) Starting with the same mass of hydrated barium chloride, suggest how you could modify the experiment to determine more accurately the mass of water lost.
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(a) Carry out the following test and record your observations.
| test | observations |
|------|--------------|
| | FA 6 | FA 7 |
| (i) Place a small spatula measure of the solid in a hard-glass test-tube and heat gently at first, then more strongly. | | |
(ii) From your observations, what is present in both salts? ..............................................................................................................................
(b) Prepare solutions of FA 6 and FA 7 by placing the rest of each solid into separate 100 cm³ beakers. Add approximately 30 cm³ of distilled water to each beaker and stir until fully dissolved. Use these solutions for tests in (b).
(i) Carry out tests to determine the cation present in each solution. Record your tests and results in the space below.
(ii) Carry out all the following tests and record your observations.
| test | observations |
|------|--------------|
| | solution of FA 6 | solution of FA 7 |
| To a 1 cm depth of solution in a test-tube add a 1 cm depth of barium chloride or barium nitrate, then | | |
| add an excess of hydrochloric acid or nitric acid. | | |
To a 1 cm depth of solution in a test-tube add a 1 cm depth of silver nitrate. | | |
(iii) Identify the ions present in each salt.
FA 6 contains ................................................. and ..................................................
FA 7 contains ................................................. and ..................................................