Chemistry 9701/31 — October/November 2013

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

questions

marks

120

minutes

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

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

In this experiment you are to determine the relative formula mass of an iron(II) salt by titration with potassium manganate(VII).

FA 1 is the iron(II) salt.
FA 2 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 3 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Preparing a solution of FA 1

Weigh the $250\text{ cm}^3$ beaker and record the mass in the space below.
Add all the FA 1 provided to the beaker. Weigh the beaker with FA 1 and record the mass.
Calculate the mass of FA 1 used and record this in the space below.
Use a measuring cylinder to add approximately $100\text{ cm}^3$ of FA 3 to the beaker. Stir until all the solid has dissolved.
Transfer the solution into the $250\text{ cm}^3$ volumetric (graduated) flask labelled FA 4.
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 the iron(II) salt is FA 4.

Titration

Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
Use a measuring cylinder to add $20\text{ cm}^3$ of FA 3 to the flask.
Fill the burette with FA 2.
Titrate FA 4 with FA 2 until the solution changes to a permanent pink colour.
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 FA 2 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 FA 4 required ........................ $\text{cm}^3$ of FA 2

(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 potassium manganate(VII) present in the volume of FA 2 calculated in (b).

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

(ii)

The half-equation for the reduction of a manganate(VII) ion is:

\text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}

Give the half-equation for the oxidation of an iron(II) ion to an iron(III) ion.

....................................................................................................................................

Therefore, 1 mole of manganate(VII) ions reacts with 5 moles of iron(II) ions.

(iii)

Calculate the number of moles of iron(II) ions present in $25.0\text{ cm}^3$ of solution FA 4.

moles of $\text{Fe}^{2+}$ in $25.0\text{ cm}^3$ of FA 4 = ....................... mol

(iv)

Calculate the number of moles of iron(II) ions present in $250\text{ cm}^3$ of solution FA 4.

moles of $\text{Fe}^{2+}$ in $250\text{ cm}^3$ of FA 4 = ....................... mol

(v)

In 1 mole of the iron(II) salt, FA 1, there is 1 mole of iron(II) ions. Use the mass of FA 1 you weighed out to calculate the relative formula mass of the iron(II) salt.

relative formula mass = .......................

(d)

(i)

A $25\text{ cm}^3$ pipette is accurate to $\pm0.06\text{ cm}^3$ .
Calculate the maximum percentage error when the pipette was used to measure solution FA 4.

percentage error in measuring FA 4 = ...................... %

(ii)

State the maximum error in the mass of the $250\text{ cm}^3$ beaker that you recorded in (a).

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

(iii)

Calculate the maximum percentage error in the mass of FA 1 used in (a).

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

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation13M

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