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You are provided with the following reagents.
• FB 1, hydrated copper(II) sulfate
• FB 2, aqueous copper(II) sulfate
• FB 3, aqueous sodium thiosulfate
• FB 4, aqueous potassium iodide
• FB 5, starch indicator solution
The formula of hydrated copper(II) sulfate is $\text{CuSO}_4\cdot x \text{H}_2\text{O}$ where $x$ shows the number of molecules of water of crystallisation present.
The value of $x$ can be found by two different methods.
Method 1 involves heating to drive off water of crystallisation while Method 2 uses a titration to determine the concentration of $\text{Cu}^{2+}(\text{aq})$.
(a) Method 1
• Weigh a crucible and record the mass.
• Add between 2.50 g and 2.70 g of FB 1 and record the new mass.
• Place the crucible containing FB 1 on a pipe clay triangle and heat gently for about four minutes with a Bunsen burner.
• Allow the crucible to cool. You should continue with Method 2 while the crucible is cooling.
• Weigh the crucible and its contents.
Record all masses in the space below.
(b) Calculate the mass of water lost and the mass of copper(II) sulfate that remained after heating.
mass of water lost = …………….. g
mass of copper(II) sulfate remaining = …………….. g [3]
(c) Use your answer to (b) to calculate how many moles of water were lost and the moles of copper(II) sulfate, $\text{CuSO}_4$, remaining after heating. Show all of your working.
[Ar: Cu, 63.5; H, 1.0; O, 16.0; S, 32.1]
The hydrated copper(II) sulfate contained ………………………….. mol of water
and ………………………….. mol of $\text{CuSO}_4$. [2]
(d) Use your answer to (c) to determine the value of $x$ in the formula of hydrated copper(II) sulfate, $\text{CuSO}_4\cdot x \text{H}_2\text{O}$.
$x$ = ……………….. [2]
(e) Method 2
• Fill the burette with FB 3, aqueous sodium thiosulfate.
• Pipette 25.0 $\text{cm}^3$ of FB 2 into a conical flask and use a measuring cylinder to add 10 $\text{cm}^3$ of FB 4.
• Titrate this solution with FB 3 from the burette until the mixture becomes yellow-brown. Do not add too much FB 3 at this stage.
• An off-white precipitate is also present in the flask and this will mask the colour of the solution.
• Add approximately 1 $\text{cm}^3$ of FB 5. The solution will become blue-black as a starch iodine complex is formed.
• Continue the titration until the blue-black colour of the complex just disappears leaving the off-white precipitate.
• Perform sufficient further titrations to obtain accurate results.
• Record your titration results in the space below. Make certain that your recorded results show the precision of your working.
Summary
25.0 $\text{cm}^3$ of FB 2 reacted with ……………….. $\text{cm}^3$ of FB 3.
Show which results you used to obtain the value of the volume of FB 3 by placing a tick (✓) under the readings used in your results. [11]
(f) (i) In Method 1 a student was advised to carry out all weighings using the same balance. What type of error might be introduced if more than one balance was used?
.........................................................................................................................
.........................................................................................................................[1]
(ii) In Method 2, 10 $\text{cm}^3$ of FB 4 was added during the titration process. Assume that the measuring cylinder used could be read to ± 0.5 $\text{cm}^3$. Calculate the percentage error in the measurement of this volume.
………….. % error [1]
(g) Method 1 is usually less accurate than Method 2 for finding the value of $x$ in the formula of hydrated copper(II) sulfate, $\text{CuSO}_4\cdot x \text{H}_2\text{O}$.
A group of students carried out Method 1 correctly but calculated a value of 4 for $x$. The true value for $x$ is 5.
Suggest an error in the practical procedure of the experiment that could account for this difference.
.........................................................................................................................
.........................................................................................................................[1]
(h) Suggest a modification that could be made to the practical procedure in Method 1 to reduce this error.
Explain why this modification should give an answer nearer to 5.
modification ...........................................................................................
explanation ...........................................................................................
.........................................................................................................................[2]
522
525
597
(a) Solutions FB 6, FB 7 and FB 8 are known to be either chlorides or sulfates of aluminium,
magnesium or calcium. The addition of aqueous sodium hydroxide and aqueous
ammonia can be used to give information about the cation present.
Add NaOH(aq) and NH3(aq) separately to each of the solutions FB 6, FB 7 and FB 8.
Rinse and reuse test-tubes where possible.
Record both the tests and your observations in an appropriate form in the space below.
From your observations identify the solutions containing aluminium, magnesium and
calcium ions. In each case give evidence to support your answer.
Solution ………………… contains the aluminium ion.
supporting evidence .................................................................
Solution ………………… contains the magnesium ion.
supporting evidence .................................................................
Solution ………………… contains the calcium ion.
supporting evidence ................................................................................
[7]
(b) Choose a pair of reagents that, used together, would identify which solution or solutions
contain(s) the sulfate ion.
Carry out the tests and record, in the space below, the reagents used and the
observations made.
The sulfate ion is present in .......................................... . [2]
589
565
526
(a) You are to carry out the tests given in the table below on solid FB 9.
Record details of any gases that are released in the reactions. These gases should be identified by a test, described in the appropriate part of the table.
No additional tests should be attempted.
| test | observations |
| --- | --- |
| (i) | Place a spatula measure of FB 9 in the small hard-glass test-tube labelled FB 9 and heat the solid strongly. |
| (ii) | To 1 cm depth of aqueous sodium hydroxide in a boiling-tube, add 1 spatula measure of FB 9, then
gently heat the mixture, do not boil
Care is needed when heating aqueous sodium hydroxide. |
| (iii) | To 1 cm depth of aqueous sodium hydroxide in a boiling-tube, add 1 spatula measure of FB 9 and a piece of aluminium foil, then
gently heat the mixture.
Care is needed when heating aqueous sodium hydroxide. |[4]
(b) What elements must be present in FB 9 to give the results you have obtained in test (i) and test (iii)?
...................................................................................................................................... [1]
(c) What is the function of the aluminium foil in test (iii)?
...................................................................................................................................... [1]
(d) Do not carry out this test
What would you expect to see if 1 cm depth of dilute hydrochloric acid was added to a spatula measure of FB 9?
...................................................................................................................................................
................................................................................................................................................... [1]
557
506
540
