Chemistry 9701/31 — October/November 2015

Cambridge AS Level · Advanced Practical Skills 1 · 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 will determine the ionic equation for the reaction of acidified potassium manganate(VII) with potassium iodide. Excess potassium iodide is used and the reaction produces iodine. The amount of iodine produced is measured by titration with sodium thiosulfate.

FA 1 is $0.0180\text{ mol dm}^{-3}$ potassium manganate(VII), KMnO₄.
FA 2 is $1.00\text{ mol dm}^{-3}$ sulfuric acid, H₂SO₄.
FA 3 is $0.500\text{ mol dm}^{-3}$ potassium iodide, KI.
FA 4 is $0.100\text{ mol dm}^{-3}$ sodium thiosulfate, Na₂S₂O₃.
starch indicator

(a)

Method

● Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
● Use the measuring cylinder to add $25\text{ cm}^3$ of FA 2 to the conical flask.
● Use the measuring cylinder to add $20\text{ cm}^3$ of FA 3 to the conical flask.
● Fill the burette with FA 4.
● Carry out a rough titration. When the colour of the mixture becomes yellow/orange, add a few drops of starch indicator. Then titrate until the mixture goes colourless.
● Record all 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 4 added in each accurate titration.

Keep FA 1 and FA 2 for use in Question 3 and FA 4 for use in Question 2.

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations.
Show clearly how you have obtained this value.

Volume of FA 4 required is ..................... $\text{cm}^3$ .

(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 in the volume of FA 4 calculated in (b).

moles of $\text{Na}_2\text{S}_2\text{O}_3$ = ............................. mol

(ii)

Use the equation below to calculate the number of moles of iodine that reacted with the sodium thiosulfate in the titration.

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

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

(iii)

Use information on page 2 to calculate the number of moles of potassium manganate(VII) in FA 1 used in the titration.

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

(iv)

From your answers to (ii) and (iii), calculate the number of moles of iodine produced by the reaction of $2.00\text{ moles}$ of potassium manganate(VII) with excess potassium iodide.

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

(v)

Using your answer to (iv), put a tick next to the ionic equation that represents the reaction between FA 1 and FA 3.

\begin{aligned} &2\text{MnO}_4^- + 2\text{I}^- + 16\text{H}^+ \rightarrow \text{I}_2 + 2\text{Mn}^{6+} + 8\text{H}_2\text{O} &\dots\dots\dots\\ &2\text{MnO}_4^- + 4\text{I}^- + 16\text{H}^+ \rightarrow 2\text{I}_2 + 2\text{Mn}^{5+} + 8\text{H}_2\text{O} &\dots\dots\dots\\ &2\text{MnO}_4^- + 6\text{I}^- + 16\text{H}^+ \rightarrow 3\text{I}_2 + 2\text{Mn}^{4+} + 8\text{H}_2\text{O} &\dots\dots\dots\\ &2\text{MnO}_4^- + 8\text{I}^- + 16\text{H}^+ \rightarrow 4\text{I}_2 + 2\text{Mn}^{3+} + 8\text{H}_2\text{O} &\dots\dots\dots\\ &2\text{MnO}_4^- + 10\text{I}^- + 16\text{H}^+ \rightarrow 5\text{I}_2 + 2\text{Mn}^{2+} + 8\text{H}_2\text{O} &\dots\dots\dots\\ &2\text{MnO}_4^- + 12\text{I}^- + 16\text{H}^+ \rightarrow 6\text{I}_2 + 2\text{Mn}^+ + 8\text{H}_2\text{O} &\dots\dots\dots \end{aligned}

(vi)

Prove that the iodide ion has been oxidised in the equation that you selected in (v).

(d)

(i)

The error in calibration of the pipette you used is $\pm 0.06\text{ cm}^3$ .
Calculate the percentage error when measuring FA 1, using the pipette.

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

(ii)

A student suggested that the experiment would be more accurate if a pipette was used to measure solution FA 3.
State and explain whether you agree with the student.

The rest of this paper

2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation9M
Q3Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation16M

Loading the full paper…