AS & A Level Chemistry - 9701 Paper 3 2013 Winter Zone 3

In this experiment you are to determine the relative formula mass of an iron(II) salt by titration with potassium manganate(VII).
FA 1 is the iron(II) salt.
FA 2 is 0.0200 mol dm⁻³ potassium manganate(VII), KMnO₄.
FA 3 is dilute sulfuric acid, H₂SO₄.

(a) Method

Preparing a solution of FA 1

• Weigh the 250 cm³ beaker and record the mass in the space below.
• Add all the FA 1 provided to the beaker. Weigh the beaker with FA 1 and record the mass.
• Calculate the mass of FA 1 used and record this in the space below.
• Use a measuring cylinder to add approximately 100 cm³ of FA 3 to the beaker. Stir until all the solid has dissolved.
• Transfer the solution into the 250 cm³ volumetric (graduated) flask labelled FA 4.
• Wash out the beaker thoroughly using distilled water and add the washings to the volumetric flask. Make the solution up to the mark using distilled water.
• Shake the flask thoroughly to mix the solution before using it for your titrations.
• This solution of the iron(II) salt is FA 4.

• Pipette 25.0 cm³ of FA 4 into a conical flask.
• Use a measuring cylinder to add 20 cm³ of FA 3 to the flask.
• Fill the burette with FA 2.
• Titrate FA 4 with FA 2 until the solution changes to a permanent pink 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 2 added in each accurate titration.

In this experiment you are to determine the formula of hydrated barium chloride, FA 5, by heating to remove the water of crystallisation. You will heat two separate samples. The anhydrous barium chloride does not decompose when heated.

FA 5 is hydrated barium chloride, $\text{BaCl}_2\cdot x\text{H}_2\text{O}$.

(a) Method

Record all weighings, in an appropriate form, in the space below.

• Record the mass of the empty crucible without its lid.
• Add between 2.0 and 2.4 g of FA 5 into the crucible. Record the mass of the crucible and its contents.
• Use a pipe-clay triangle to support the crucible and contents on a tripod.
• Heat the crucible and its contents gently for about one minute with the lid off. Then heat strongly for a further four minutes.
• Put the lid on the crucible and leave to cool for approximately 10 minutes.

While you are waiting for the crucible to cool, start work on Question 3.
• When the crucible is cool, remove the lid, and weigh the crucible with the residue.
• Record the mass of anhydrous barium chloride remaining in the crucible after heating and the mass of water lost.
• To prepare for the second experiment, use a spatula to remove the residue from the crucible into the beaker labelled waste.
• Reweigh the empty crucible without its lid.
• Carry out the experiment again. This time use between 1.5 and 1.9 g of FA 5.
[6]

(b) Calculation

Show your working in each step.

(i) Calculate the mean number of moles of water removed from the hydrated salt in the experiments.
($A_r$: H, 1.0; O, 16.0)

moles of $\text{H}_2\text{O} = \text{.................... mol}$

(ii) Calculate the mean number of moles of anhydrous barium chloride produced in the experiments.
($A_r$: Ba, 137; Cl, 35.5)

moles of $\text{BaCl}_2 = \text{............. mol}$

(iii) Calculate the value of x in the formula of hydrated barium chloride, $\text{BaCl}_2\cdot x\text{H}_2\text{O}$.

x = ................
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(c) (i) Suggest how the experimental procedure could be modified to ensure that all of the water of crystallisation had been removed by heating hydrated FA 5.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
(ii) Do you think that the results from your two experiments are consistent with each other?
Justify your answer by carrying out appropriate calculations.
[3]

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) FA 5 is hydrated barium chloride.
FA 6 is the same iron(II) salt used in Question 1. It contains one other cation and one anion.
(i) Place a small spatula measure of FA 6 into a test-tube. Dissolve the solid in about 5 cm depth of distilled water. Use the solution for the following tests.

[Table_1: test | observations]
To a 1 cm depth of aqueous FA 6 in a boiling tube, add aqueous sodium hydroxide until no further change occurs, then
..............................................................................
heat the mixture carefully.

Dissolve a few crystals of FA 5 in a 1 cm depth of distilled water in a test-tube. Add a 1 cm depth of FA 6, then
..............................................................................
add excess dilute hydrochloric acid to the mixture.

(ii) Identify the ions present in FA 6.
cations: Fe^{2+} and .......................................... anion: .................................................

(iii) Give the ionic equation for the reaction of iron(II) ions with hydroxide ions.
....................................................................................................................................

(iv) Place a small spatula measure of FA 6 into a hard-glass test-tube. Heat gently, then strongly, until no further change is observed. Record your observations in the space below.

(b) FA 7, FA 8 and FA 9 are aqueous solutions. Each contains one cation and one anion. FA 3 is dilute sulfuric acid, H_{2}SO_{4}.
Mix pairs of solutions so that you can complete the table below. For each test, use 1 cm depths of each solution in clean test-tubes. Record your observations in the table.

[Table_2: FA 7 | FA 8 | FA 9]
[FA 3 | ]
[FA 7 | X]
[FA 8 | X]

From your observations, complete the following statements.
The anion in FA 7 is .........................................
The cation in FA 8 is .........................................
The anion in FA 9 is .........................................