Chemistry 9701/31 — May/June 2015

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 ObservationsFree sample

In this question you will determine the concentration of iron(II) ions in FA 2. To do this you will do a titration using potassium manganate(VII) solution. The iron(II) ions, $\text{Fe}^{2+}$ , are oxidised by the manganate(VII) ions, $\text{MnO}_4^-$ .

5\text{Fe}^{2+}(\text{aq}) + \text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) \rightarrow 5\text{Fe}^{3+}(\text{aq}) + \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})

When all the $\text{Fe}^{2+}$ ions have been oxidised, the presence of unreacted $\text{MnO}_4^-$ ions causes the solution to become a permanent pink colour.

FA 1 contains $0.0200\text{ mol dm}^{-3}$ manganate(VII) ions, $\text{MnO}_4^-$ .
FA 2 is a solution containing iron(II) ions, $\text{Fe}^{2+}$ .
FA 3 is $1.0\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Fill the burette with FA 1.
Use the pipette to transfer $25.0\text{ cm}^3$ of FA 2 into the conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to add $10\text{ cm}^3$ of FA 3 to the conical flask.
Add FA 1 from the burette into the conical flask until the solution becomes a permanent pink colour.
Perform a rough titration and record your burette readings in the space below.

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

Do as many accurate titrations as you think necessary to obtain consistent results.
Make certain 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 1 added in each accurate titration.

Keep FA 2 to use in Question 3.

(b)

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

$25.0\text{ cm}^3$ of FA 2 required ......................... $\text{cm}^3$ of FA 1.

(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 manganate(VII) ions present in the volume of FA 1 calculated in (b).

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

(ii)

Calculate the number of moles of iron(II) ions present in $25.0\text{ cm}^3$ of FA 2.

moles of $\text{Fe}^{2+}$ = ............................. mol

(iii)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of iron(II) ions in FA 2.

concentration of $\text{Fe}^{2+}$ in FA 2 = ............................. $\text{mol dm}^{-3}$

(iv)

FA 2 was prepared by dissolving hydrated ammonium iron(II) sulfate, $(\text{NH}_4)_2\text{Fe}(\text{SO}_4)_2\cdot6\text{H}_2\text{O}$ in distilled water. Calculate the mass of salt that would have to be dissolved in $1.00\text{ dm}^3$ of water to prepare FA 2.
( $A_r$ : $\text{H}, 1.0$ ; $\text{N}, 14.0$ ; $\text{O}, 16.0$ ; $\text{S}, 32.1$ ; $\text{Fe}, 55.8$ )

mass of $(\text{NH}_4)_2\text{Fe}(\text{SO}_4)_2\cdot6\text{H}_2\text{O}$ = ............................. g

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation15M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation2M

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