AS & A Level Chemistry - 9701 Paper 3 2009 Summer Zone 1

You are provided with the following.
FA 1 is 0.15 mol dm$^{-3}$ sodium thiosulfate, Na$_2$S$_2$O$_3$.
FA 2 is aqueous copper(II) sulfate.
You are also provided with a 10% solution of potassium iodide, KI, and starch indicator.

You are required to determine the concentration, in g dm$^{-3}$, of hydrated copper(II) sulfate, CuSO$_4$.5H$_2$O, in FA 2.

Dilution of FA 2

(a) By using a burette measure between 47.00 cm$^{3}$ and 47.50 cm$^{3}$ of FA 2 into the 250 cm$^{3}$ graduated flask labelled FA 3.
Record your burette readings and the volume of FA 2 added to the flask in the space below.

[Space provided in the image]
Make up the contents of the flask to the 250 cm$^{3}$ mark with distilled water. Place the stopper in the flask and mix the contents thoroughly by slowly inverting the flask a number of times.

Titration
Fill a second burette with FA 1.

Perform a rough (trial) titration as follows.

Pipette 25.0 cm$^{3}$ of FA 3 into a conical flask.
Use the measuring cylinder provided to add 10 cm$^{3}$ of 10% potassium iodide to the flask.
The Cu$^{2+}$ ions in FA 3 oxidise the iodide ions to iodine, I$_2$, which can be titrated with FA 1.
The flask will also contain an off-white precipitate of copper(I) iodide, CuI.

Run FA 1 from the burette, 1 cm$^{3}$ at a time, until the brown colour of the iodine solution has changed to pale brown.
Add approximately 10 drops of starch indicator. A blue-black colour should be seen as the starch reacts with the residual iodine.
Continue to add FA 1 1 cm$^{3}$ at a time until the blue-black colour of the starch-iodine complex disappears and there is no further colour change.

In this rough titration .............. cm$^{3}$ of FA 1 were added.

Perform sufficient further titrations to obtain reliable results.
Record your titration results in the space below. Make certain that your recorded results show the precision of your working.

[6]

(b) From your titration results obtain a volume of FA 1 to be used in your calculations.
Show clearly how you obtained this volume.

[1]

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

(c) Use your answer to (b) to calculate how many moles of Na$_2$S$_2$O$_3$ were run from the burette into the conical flask.

...... mol of Na$_2$S$_2$O$_3$ were run from the burette into the conical flask.
Calculate how many moles of I$_2$ reacted with the Na$_2$S$_2$O$_3$ run from the burette.

$2$S$_2$O$_3^{2-}$ $\rightarrow$ S$_4$O$_6^{2-}$ + $2e^{-}$
I$_2$ + $2e^{-}$ $\rightarrow$ 2I$^{-}$

...... mol of I$_2$ reacted with the Na$_2$S$_2$O$_3$ run from the burette.
Calculate how many moles of Cu$^{2+}$ ions reacted with iodide ions to produce this amount of I$_2$.

$2$Cu$^{2+}$ + $4$I$^{-}$ $\rightarrow$ $2$CuI + I$_2$

...... mol of Cu$^{2+}$ reacted to form the I$_2$.
Calculate the concentration, in mol dm$^{-3}$, of Cu$^{2+}$ in FA 3.

The concentration of Cu$^{2+}$ in FA 3 is ...... mol dm$^{-3}$.
Calculate the concentration, in mol dm$^{-3}$, of Cu$^{2+}$ in FA 2.

The concentration of Cu$^{2+}$ in FA 2 is ...... mol dm$^{-3}$.
Calculate the concentration, in g dm$^{-3}$, of CuSO$_4$.5H$_2$O in FA 2.
[A$_r$: Cu, 63.5; H, 1.0; O, 16.0; S, 32.1]

FA 2 contains ...... g dm$^{-3}$ CuSO$_4$.5H$_2$O.
[5]

(d) The maximum error in any burette reading is $\pm$0.05 cm$^{3}$.
Explain how the maximum error in a titration is therefore $\pm$0.10 cm$^{3}$.

[1]

(e) Calculate the maximum percentage error in the average titre given in (b).
The error is ......................................%.
[1]

You are to investigate how the rate of formation of sulfur varies with the concentration of sodium thiosulfate, Na₂S₂O₃, in the reaction below.

Na₂S₂O₃(aq) + 2HCl(aq) → S(s) + 2NaCl(aq) + SO₂(g) + H₂O(l)

Care should be taken to avoid inhalation of SO₂(g) that is given off during this reaction.

You are provided with the following.
FA 1, 0.15 mol dm⁻³ Na₂S₂O₃  a measuring cylinder to measure 50 cm³
FA 4, 2.0 mol dm⁻³ HCl    a measuring cylinder or marked tube to measure 5 cm³
a printed insert    a stop clock or clock with seconds hand

(a) Method – Read through the instructions before starting any practical work.

• Using the larger measuring cylinder transfer 50 cm³ of FA 1 into a 250 cm³ beaker.
• Measure 5 cm³ of FA 4 in the smaller measuring cylinder (or marked tube).
• Tip the FA 4 into the FA 1 in the beaker and immediately start timing.
• Swirl the beaker to mix the solution and place it on top of the printed insert.
• View the printed insert from above so that it is seen through the solution.
• Note the time when the printing on the insert just disappears.

• Empty and rinse the beaker. Shake out as much of the rinse water as possible and dry the outside of the beaker.
• Repeat the experiment using 25 cm³ of FA 1 and 25 cm³ of distilled water. Add 5 cm³ of FA 4 to start the reaction.

• Select suitable volumes of FA 1 and distilled water for one further experiment to investigate the effect of sodium thiosulfate concentration on the rate of reaction. Remember to use 5 cm³ of FA 4 and to keep the total volume of FA 1 and distilled water constant.

In an appropriate form record the following below:
• all measurements of volume and time (to the nearest second) for each experiment,
• calculated values of 1/ₑᵢₙ which are a measure of the rate of reaction.

Results

[9 marks]

(b) The total volume in each experiment is constant. Using volumes from the first two experiments, show by simple calculation that the volume of FA 1 used is a measure of its concentration in the reaction mixture.

[1 mark]

(c) What is the relationship between the rate of reaction and the time taken?
..........................................................
..........................................................
..........................................................

[1 mark]

(d) For each experiment calculate the numerical value of (volume of FA 1 × time).

(a) Carry out the following tests. Record your observations in the spaces provided in the table.

[Table_1]

To 1 cm depth of solution in a test-tube add aqueous sodium hydroxide, a little at a time, until in excess.
To 1 cm depth of solution in a test-tube add aqueous ammonia, a little at a time, until in excess.

Using the qualitative analysis notes printed on page 11 and the observations above it is possible to identify the cation present in one of the solutions and also to identify possible cations in another of the solutions.

Solution ...................... contains the single cation ..................................................
Solution ...................... contains one of the following cations, ..................................... .

(b) You are to select suitable reagents and carry out tests on the solutions to identify which solution or solutions contain either a nitrate or a nitrite ion.

Record in an appropriate form below the tests performed and the observations made.

Nitrate or nitrite ions are contained in solution(s) .......................................... .

(c) Carry out the following tests.

[Table_2]

To 1 cm depth of solution in a test-tube add 1 cm depth of dilute hydrochloric acid.

Use these observations to identify the cation or anion present in each solution and complete the table below.

[Table_3]

(d) FA 5 and FA 7 can be mixed to confirm the identity of one ion in each of the two solutions.

[Table_4]

This observation confirms the presence of .............. in FA 5 and .............. in FA 7.