AS & A Level Chemistry - 9701 Paper 3 2011 Summer Zone 3

FA 1 is an iron salt in which all the iron is present as $\text{Fe}^{2+}$ cations. You will work out the percentage of iron in this salt by titrating a solution of this salt with a standard solution of aqueous potassium manganate(VII).

FA 1 is an unknown iron(II) salt.
FA 2 is $1.00\, \text{mol} \cdot \text{dm}^{-3}$ sulfuric acid.
FA 3 is $0.0100\, \text{mol} \cdot \text{dm}^{-3}$ potassium manganate(VII).

(a) Method

Weighing out the salt

• Weigh the tube containing FA 1.
• Tip the contents of the tube into a $250\, \text{cm}^3$ beaker.
• Re-weigh the empty tube.
• Record all your readings in a suitable form in the space below.

Preparing the solution

• To the salt in the beaker use a measuring cylinder to add approximately $200\, \text{cm}^3$ of FA 2 and stir until the salt has dissolved.
• Pour the contents of the beaker carefully into the $250\, \text{cm}^3$ graduated (volumetric) flask using the small funnel.
• Rinse the contents of the beaker twice with a little distilled water and add these washings to the graduated flask.
• Fill the graduated flask to the line with distilled water. Shake carefully to ensure adequate mixing.

Titration

• Fill the burette with FA 3.
• Pipette $25.0\, \text{cm}^3$ of the solution of FA 1 from the graduated flask into a conical flask.
• Titrate the solution of FA 1 in the flask with FA 3 until the first appearance of a permanent pink colour.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

$25.0\, \text{cm}^3$ of the solution of FA 1 required $\text{....................... cm}^3$ of FA 3.

(c) Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate how many moles of $\text{MnO}_4^-(\text{aq})$ were present in the volume of FA 3 calculated in (b).

moles of $\text{MnO}_4^-(\text{aq}) = ............ \text{mol}$

(ii) Use the following equation to calculate how many moles of $\text{Fe}^{2+}(\text{aq})$ were present in the conical flask.

$\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{Fe}^{2+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 5\text{Fe}^{3+}(\text{aq}) + 8\text{H}_2\text{O}(\text{aq})$

moles of $\text{Fe}^{2+}(\text{aq})$ in the conical flask = $\text{.............. mol}$

(iii) Calculate the number of moles of $\text{Fe}^{2+}$ in your weighed sample of FA 1.

moles of $\text{Fe}^{2+}$ in the weighed sample = $\text{.............. mol}$

(iv) Calculate the percentage of iron in FA 1.

[$A_r$: Fe, 55.8]

the percentage of iron in FA 1 = $\text{............ %}$

(d) There are a number of sources of potential error in this experiment. One of these involves the readings taken using the balance.

(i) State the maximum individual error in any single balance reading.

maximum individual error = .................... g

(ii) Calculate the maximum percentage error in the mass of FA 1 used in your experiment.

maximum percentage error = .................... %

FA 4 is an impure sample of hydrated magnesium sulfate, $\text{MgSO}_4.7\text{H}_2\text{O}$. When heated the water of crystallisation is driven off to leave anhydrous magnesium sulfate, $\text{MgSO}_4$. The impurity does not give off water when heated. By determining how much water is present in the impure sample, the percentage purity can be calculated.
(a) Method

• Weigh a clean dry crucible.
• Empty all of the FA 4 into the crucible.
• Reweigh the crucible and its contents.
• Support the crucible in the pipe-clay triangle on top of a tripod.
• Heat the crucible gently for about 1 minute and then more strongly for a further 4 minutes.
• Allow the crucible to cool. You should start question 3 while cooling is taking place.
• When the crucible is cool enough to handle, reweigh the crucible and its contents.
• Repeat the cycle of heating and weighing as many times as you think necessary.

(a) Put a spatula measure of FA 5 into a boiling tube. Heat it gently for a few minutes and then strongly until no further changes are seen. Record your observations at each stage, in the space below.
Leave the boiling tube to cool. Do not discard the contents as they will be used later in the question. [2]
(b) Put a spatula measure of FA 6 into a test-tube. Half fill the test-tube with distilled water and dissolve the solid. You will use this solution to carry out the following tests.
• To a 1 cm depth of a solution of FA 6 in a boiling tube, add 0.5 cm depth of aqueous sodium hydroxide using a teat pipette. Heat the mixture carefully.
• To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous ammonia.
• To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous barium chloride or barium nitrate.
• To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous silver nitrate, followed by aqueous ammonia.
Record your observations for each of the tests in the space below. Identify the ions present in FA 6.
FA 6 contains ... and ... [6]
(c) Put a small spatula measure of FA 6 into a boiling tube. Taking great care, add 5 drops of concentrated sulfuric acid.
CARE: Concentrated sulfuric acid is very corrosive.
Once you have made your observations fill the boiling tube with water. Record your observations in the space below.
What type of chemical reaction occurs between FA 6 and sulfuric acid? Justify your answer. [3]
(d) To the residue in the boiling tube from (a), slowly and carefully add FA 7 to a depth of about 5 cm. Divide this solution equally into two test-tubes.
• To one test-tube add 5 drops of aqueous lead nitrate.
• To the other test-tube add 5 drops of aqueous silver nitrate. Record your observations for each test in the space below. [2]
(e) Use the information in the Qualitative Analysis Notes on pages 11 and 12 to select one test to confirm the identity of the cation in FA 7 and one test to confirm the identity of the anion in FA 7. Carry out both tests and record your observations for each of the tests in the space below. Identify the ions present in FA 7.
FA 7 contains ... and ... [3]