AS & A Level Chemistry - 9701 Paper 3 2019 Summer Zone 4

Iron wire contains impurities. You will investigate the percentage by mass of iron in a sample of iron wire.
A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron(II) sulfate.
\[ \text{Fe}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{H}_2(g) \]
You will titrate the solution of iron(II) sulfate with potassium manganate(VII) of known concentration to determine the amount of iron(II) ions present and hence the percentage by mass of iron in the wire.
You may assume the impurities do not form any products that react with potassium manganate(VII).
\[ 5\text{Fe}^{2+}(aq) + \text{MnO}_4^{-}(aq) + 8\text{H}^{+}(aq) \rightarrow 5\text{Fe}^{3+}(aq) + \text{Mn}^{2+}(aq) + 4\text{H}_2\text{O}(l) \]
FB 1 is 0.0200 mol dm^{-3} potassium manganate(VII), KMnO_4.
FB 2 is a solution of FeSO_4 prepared by reacting 6.02 g of iron wire with sulfuric acid to make 1 dm^3 of solution.
FB 3 is dilute sulfuric acid, H_2SO_4.

(a) Method

• Fill a burette with FB 1.
• Pipette 25.0 cm^3 of FB 2 into a conical flask.
• Use the measuring cylinder to transfer 25 cm^3 of FB 3 into the conical flask.
• Perform a rough titration and record your burette readings in the space below.

• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Make certain that any recorded results show the precision of your practical work.
• Record all of your burette readings and the volume of FB 1 added in each accurate titration.

In Question 1 you used dilute sulfuric acid, FB 3. You will now determine the concentration of FB 3 by measuring the temperature of its reaction with sodium hydroxide.

$2 ext{NaOH(aq)} + ext{H}_2 ext{SO}_4 ext{(aq)} \rightarrow ext{Na}_2 ext{SO}_4 ext{(aq)} + 2 ext{H}_2 ext{O(l)}$

FB 4 is 0.90 ext{mol dm}^{-3} sodium hydroxide, NaOH.

(a) Method

• Fill the second burette with FB 4.
• Support the plastic cup in the 250 cm³ beaker.
• Pipette 10.0 cm³ of FB 3 into the plastic cup.
• Place the thermometer into the FB 3. Tilt the cup so that the bulb of the thermometer is surrounded by solution. Record the temperature in the table below. This is the temperature of the solution before any FB 4 has been added.
• Add 5.00 cm³ of FB 4 from the burette to the FB 3 in the plastic cup. Stir the mixture and record the temperature reached. Record the volume of FB 4 added.
• Add a second 5.00 cm³ portion of FB 4 to the plastic cup, stir the mixture and record the temperature reached. Record the total volume of FB 4 added.
• Continue adding 5.00 cm³ portions of FB 4 until 40.00 cm³ have been added. After each addition, stir and record the temperature reached and total volume of FB 4 added.

Results

[Table: total volume of FB 4 added/cm³ | temperature of solution/°C]

(b) Plot a graph of temperature of solution on the y-axis against total volume of FB 4 added on the x-axis. Select a scale on the y-axis to include a temperature of 2.0 °C above your maximum thermometer reading. Label any points you consider to be anomalous. You will use the graph to find the volume of FB 4 needed to neutralise 10.0 cm³ of FB 3.

Draw two lines of best fit through the points on your graph, the first for the increase in temperature and the second for the decrease in temperature of the mixtures. Extrapolate the two lines so they intersect and hence determine the volume of FB 4 required to neutralise 10.0 cm³ of FB 3.

volume of FB 4 = [4]

(c) (i) Use your answer to (b) to calculate the number of moles of sodium hydroxide required to neutralise 10.0 cm³ of FB 3.

moles of NaOH = [1]

(ii) Calculate the number of moles of sulfuric acid present in 10.0 cm³ of FB 3.

moles of H₂SO₄ =
Hence calculate the concentration, in mol dm^-3, of sulfuric acid in FB 3.

concentration of H₂SO₄ in FB 3 = [1]

(d) Explain how you would use the data obtained in Question 2 to calculate the enthalpy change of neutralisation of the sulfuric acid. You do not need to carry out the calculation.

................................................................................................................
................................................................................................................
................................................................................................................
................................................................................................................ [3]

Qualitative Analysis

Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.

At each stage of any test you are to record details of the following:
• colour changes seen;
• the formation of any precipitate and its solubility in an excess of the reagent added;
• the formation of any gas and its identification by a suitable test.
You should indicate clearly at what stage in a test a change occurs.
If any solution is warmed, a boiling tube must be used.
Rinse and reuse test-tubes and boiling tubes where possible.

No additional tests for ions present should be attempted.

(a) FB 5 is a solution of a sodium salt of an organic acid.

FB 6 and FB 7 each contain one cation and one anion from those listed in the Qualitative Analysis Notes.

Carry out the following tests and record your observations. Use a separate 1 cm depth of each solution in a test-tube for each test.

[Table_1]

(b) (i) Select reagents for two further tests to help identify the cations present in FB 6 and FB 7. Record in a suitable form below the reagents you use and your observations.

[Space for answer]

(ii) Use your observations to identify as many ions as possible. Give the formula of the ion present. Write 'unknown' if you were unable to identify an ion.

[Table_2]

(iii) Write an ionic equation for any precipitation reaction observed involving FB 7. Include state symbols.

[Space for answer]