Chemistry 9701/35 — May/June 2020

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

Ethanedioic acid forms salts with Group 1 metals. In this experiment you will identify the Group 1 metal ion, $\text{Z}^+$ , present in an ethanedioate salt, $(\text{COO})_2\text{Z}_2$ . You will titrate a solution of the salt with acidified aqueous potassium manganate(VII). The equation for the reaction between manganate(VII) ions and ethanedioate ions in acidic solution is shown.

2\text{MnO}_4^-(\text{aq}) + 16\text{H}^+(\text{aq}) + 5(\text{COO})_2^{2-}(\text{aq}) \rightarrow 2\text{Mn}^{2+}(\text{aq}) + 8\text{H}_2\text{O}(\text{l}) + 10\text{CO}_2(\text{g})

FA 1 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 2 is a solution containing $8.06\text{ g dm}^{-3}$ of an ethanedioate salt, $(\text{COO})_2\text{Z}_2$ .
FA 3 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Fill the burette with FA 1.
Pipette $25.0\text{ cm}^3$ of FA 2 into a conical flask.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of FA 3 into the same conical flask.
Place the conical flask on the tripod and gauze and heat the conical flask until the temperature of the solution is approximately $70^\circ\text{C}$ .
Carefully remove the hot conical flask and place it on the white tile under the burette.
During titrations, add FA 1, slowly at first, until a permanent pale pink colour is formed. (The pink colour on initial addition may take several seconds to disappear.) If the reaction mixture turns brown, reheat it to approximately $70^\circ\text{C}$ . If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration.
Perform a rough titration (the end-point is a permanent pale pink colour) 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 all of your burette readings and the volume of FA 1 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 1 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

(i)

Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to the appropriate number of significant figures.

(ii)

Calculate the number of moles of manganate(VII) ions in the volume of FA 1 calculated in (b).

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

(iii)

Use the equation on page 2 to calculate the number of moles of ethanedioate ions in $25.0\text{ cm}^3$ of FA 2.

moles of $(\text{COO})_2^{2-}$ = .............................. mol

(iv)

Calculate the relative formula mass, $M_r$ , of the ethanedioate salt, $(\text{COO})_2\text{Z}_2$ .

$M_r$ of ethanedioate salt = ..............................

(v)

Calculate the relative atomic mass, $A_r$ , of the Group 1 metal, Z, in the ethanedioate salt.
Show your working.

$A_r$ of Z = ..............................

Hence identify Z.

Z is .......................................................... .

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation15M

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