Chemistry 9701/33 — May/June 2011

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

questions

marks

120

minutes

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

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

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 $1.00 \text{ mol dm}^{-3}$ sulfuric acid.
FA 3 is $0.0100 \text{ mol dm}^{-3}$ 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.

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

Carry out as many accurate titrations as you think are necessary to obtain consistent results.
Make certain any recorded results show the precision of your practical work.
Record in an appropriate form below all of your burette readings and the volume of FA 3 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.

$25.0 \text{ cm}^3$ 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 $\text{MnO}_4^-(\text{aq})$ = ............. 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 = ............. 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 = ............. mol

(iv)

Calculate the percentage of iron in FA 1.
[ $A_r$ : Fe, 55.8]

the percentage of iron in FA 1 = ............. %

(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 = ...................... %

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation8M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation16M

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