Chemistry 9701/36 — October/November 2013

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

In this experiment you are to determine the concentration of aqueous potassium manganate(VII), FB 3, by titration.

In the titration potassium manganate(VII) is first reacted with acidified potassium iodide to produce iodine. The amount of iodine formed is then determined by titrating the mixture with sodium thiosulfate.

FB 1 is hydrated sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$ .
FB 2 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .
FB 3 is aqueous potassium manganate(VII), $\text{KMnO}_4$ .
FB 4 is aqueous potassium iodide, $\text{KI}$ .
starch indicator

(a)

Method

Preparing a solution of FB 1

Weigh the $250\text{ cm}^3$ beaker and record the mass in the space below.
Add all the FB 1 to the beaker. Weigh the beaker with FB 1 and record the mass.
Calculate the mass of FB 1 used and record this in the space below.
Add approximately $100\text{ cm}^3$ of distilled water to the beaker. Stir until all the solid has dissolved.
Transfer the solution into the $250\text{ cm}^3$ volumetric (graduated) flask labelled FB 5.
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 sodium thiosulfate is FB 5.

Titration

Use the measuring cylinder to add $20\text{ cm}^3$ of FB 2 to a conical flask.
Use the measuring cylinder to add $10\text{ cm}^3$ of FB 4 to the same flask.
Pipette $25.0\text{ cm}^3$ of FB 3 into the same flask. The colour of the mixture is caused by iodine.
Fill the burette with FB 5.
Begin each titration without adding the starch indicator.
Add 10 drops of starch indicator when the colour of the mixture becomes (pale) yellow.
The end-point is when the blue-black colour caused by the starch disappears.
Perform a rough titration and record your burette readings in the space below.

The rough titre is ........................ $\text{cm}^3$ .

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 FB 5 added in each accurate 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.

suitable value = ................ $\text{cm}^3$ of FB 5

(c)

Calculations

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

(i)

Calculate the number of moles of sodium thiosulfate, FB 1, that were weighed out. The relative formula mass of hydrated sodium thiosulfate is 248.2.

moles of sodium thiosulfate = ....................... mol

(ii)

Calculate the number of moles of sodium thiosulfate that were present in the volume of FB 5 calculated in (b).

moles of sodium thiosulfate = ....................... mol

(iii)

Iodine produced by the reaction in the conical flask reacts with sodium thiosulfate. Use the equation below to calculate the number of moles of iodine that reacted with sodium thiosulfate in (ii).

\text{I}_2 + 2\text{Na}_2\text{S}_2\text{O}_3 \rightarrow 2\text{NaI} + \text{Na}_2\text{S}_4\text{O}_6

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

(iv)

The iodine is produced as a result of the oxidation of iodide ions in potassium iodide, FB 4, by potassium manganate(VII), FB 3. The ionic equation for this reaction is

2\text{MnO}_4^- + 16\text{H}^+ + 10\text{I}^- \rightarrow 2\text{Mn}^{2+} + 5\text{I}_2 + 8\text{H}_2\text{O}

Calculate the number of moles of potassium manganate(VII), $\text{KMnO}_4$ , that reacted to produce the iodine in (iii).

moles of $\text{KMnO}_4$ = ...................... mol

(v)

Calculate the concentration of potassium manganate(VII), in $\text{g dm}^{-3}$ , in FB 3.
( $A_r$ : $\text{O} = 16.0$ ; $\text{K} = 39.1$ ; $\text{Mn} = 54.9$ )

concentration of $\text{KMnO}_4$ = ..................... $\text{g dm}^{-3}$

(d)

(i)

State the maximum error in any single reading of the burette.

maximum error = .................. $\text{cm}^3$

(ii)

Calculate the maximum percentage error in volume of FB 5 in your first accurate titre.

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

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