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Read through question 1 before starting any practical work.
You are provided with the following reagents.
• FA 1, hydrated iron(II) sulfate
• FA 2, aqueous iron(II) sulfate
• FA 3, aqueous potassium manganate(VII)
• FA 4, sulfuric acid
The formula of hydrated iron(II) sulfate is $FeSO_4.xH_2O$ 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 $Fe^{2+}$(aq).
(a) Method 1
• Weigh a crucible and record the mass.
• Add between 1.80 g and 2.00 g of FA 1 and record the new mass.
• Place the crucible containing FA 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.
[3]
(b) Calculate the mass of water lost and the mass of iron(II) sulfate that remained after heating.
mass of water lost = …………… g
mass of iron(II) sulfate remaining = …………… g [1]
(c) Use your answer to (b) to calculate how many moles of water were lost and the moles of iron(II) sulfate, $FeSO_4$, remaining after heating. Show all of your working.
[Ar : Fe, 55.8; H, 1.0; O, 16.0; S, 32.1]
The hydrated iron(II) sulfate contained …………………………… mol of water and …………………………… mol of $FeSO_4$. [2]
(d) Use your answer to (c) to determine the value of $x$ in the formula of hydrated iron(II) sulfate, $FeSO_4.xH_2O$.
$x$ = ……………….. [2]
(e) Method 2
• Fill the burette with FA 3, aqueous potassium manganate(VII).
• Pipette 25.0 $cm^3$ of FA 2 into a conical flask and use a measuring cylinder to add approximately 20 $cm^3$ of FA 4.
• Titrate this solution with FA 3 from the burette until the first permanent pink colour remains in the solution.
• 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 $cm^3$ of FA 2 reacted with ………………… $cm^3$ of FA 3.
Show which results you used to obtain the value of the volume of FA 3 by placing a tick (✓) under the readings used in your results. [11]
(f) All experimental methods contain errors, some of which are concerned with uncertainty of measurements.
Complete the table below to show the uncertainties in measuring the volume of potassium manganate(VII) used in Method 2.
```
| maximum uncertainty in a single reading with a burette | ± ……………….. $cm^3$ |
| volume of potassium manganate(VII), FA 3, from the summary in (e) | ……………… $cm^3$ |
| maximum percentage error in the volume of potassium manganate(VII) used | …………….. % |
```
[2]
(g) Method 1 is usually less accurate than Method 2 for finding the value of $x$ in the formula of hydrated iron(II) sulfate, $FeSO_4.xH_2O$.
A group of students carried out Method 1 correctly but calculated a value of 9 for $x$. The true value for $x$ is 7.
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 7.
modification .................................................................
......................................................................................
explanation ..............................................................
.................................................................
[2]
[Total: 24]
546
568
570
The four solutions FA 5, FA 6, FA 7 and FA 8 each contain one of the following anions.
• chloride, $Cl^-$
• iodide, $I^-$
• nitrate, $NO_3^-$
• nitrite, $NO_2^-$
Use information from the Qualitative Analysis Notes on page 11 to answer the following questions.
(a) Which single reagent could you use to identify the solution containing the nitrite ion?
............................................................................................................................
Which single reagent could you use to identify the solutions containing the chloride and the iodide ion?
............................................................................................................................[1]
(b) Use the reagents selected in (a) to test each of the solutions.
Rinse and reuse test-tubes where possible.
Record in an appropriate form in the space below, the reagents used and the observations made.
From your observations identify the solutions containing chloride, iodide and nitrite ions.
In each case give evidence to support your answer.
solution ................. contains the chloride ion.
supporting evidence .......................................................
solution ................. contains the iodide ion.
supporting evidence .......................................................
solution ................. contains the nitrite ion.
supporting evidence .......................................................[7]
(c) Do not carry out this test.
State another test that you could use to confirm the presence of chloride and iodide ions.
............................................................................................................................[1]
594
513
516
(a) You are to carry out the tests given in the table below on solutions FA 9 and FA 10. You should record details of colour changes seen and the formation of any precipitate.
No additional tests should be attempted.
Reheat your water bath until the water boils. Turn off the Bunsen burner.
test | observations
[Table_1]
(i) To 1 cm depth of FA 9 in a test-tube, add 1 cm depth of dilute hydrochloric acid.
(ii) To 1 cm depth of FA 9 in a test-tube, add 1 cm depth of dilute sulfuric acid.
(iii) To 1 cm depth of FA 10 in a boiling-tube, add dilute sulfuric acid until no further change occurs.
Use this solution for test (iv).
(iv) To the solution left after test (iii) add 1 cm depth of ethanol.
Place the mixture in your hot water bath and leave for approximately 3 minutes.
(v) To 1cm depth of FA 9 in a test-tube add 1 cm depth of FA 10.
(b) From your observations in (a) identify the cation present in FA 9. Give evidence to support your answer.
Solution FA 9 contains the ........... ion.
supporting evidence .................................................................................................
.................................................................................................................................... [2]
(c) What chemical change, involving ethanol, takes place in reaction (iv)? Give evidence to support your answer.
chemical change .....................................................................................................
supporting evidence .................................................................................................
.................................................................................................................................... [1]
596
538
572
