Chemistry 9701/36 — October/November 2017

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

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marks

120

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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

FB 1 is a solution made by dissolving an unknown mass of a mixture of ethanedioic acid, $(\text{COOH})_2$ , and sodium ethanedioate, $(\text{COONa})_2$ . You will carry out two titrations to find the percentage by mass of ethanedioic acid in the mixture.

Titration 1

In aqueous solution both ethanedioic acid and sodium ethanedioate release all their ethanedioate ions, $(\text{COO}^-)_2$ . These ions react with manganate(VII) ions as shown.

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

FB 1 is an aqueous solution of the mixture containing ethanedioic acid and sodium ethanedioate.
FB 2 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FB 3 is $1.00\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Fill a burette with FB 2.
Pipette $25.0\text{ cm}^3$ of FB 1 into a conical flask.
Use the measuring cylinder to add $30\text{ cm}^3$ of FB 3 to the same conical flask.
Place the conical flask on the tripod and gauze and heat until the solution is at a temperature of approximately $70\text{ }^\circ\text{C}$ .
Carefully remove the flask from the tripod and place it under the burette, ready for the titration.
Add FB 2 from the burette, slowly at first, until a permanent pale pink colour is formed. If the reaction mixture turns brown, reheat it to about $70\text{ }^\circ\text{C}$ . 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 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 FB 2 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations.

Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FB 1 required ............................. $\text{cm}^3$ of FB 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 manganate(VII) ions in the volume of FB 2 calculated in (b).

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

(ii)

Calculate the total number of moles of ethanedioate ions present in $25.0\text{ cm}^3$ of FB 1.

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

(d)

Titration 2

Ethanedioic acid reacts with aqueous sodium hydroxide. In this reaction both the $\text{H}^+$ ions formed by the acid molecule react.

Complete the equation showing the reaction between ethanedioic acid and sodium hydroxide including state symbols.

........ $(\text{COOH})_2(\text{aq})$ + ........ $\text{NaOH}(\text{aq}) \rightarrow$ .......................... + ..........................

(e)

FB 4 is $0.0400\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
thymol blue indicator

Method

Fill the second burette with FB 4.
Pipette $25.0\text{ cm}^3$ of FB 1 into a conical flask.
Add about 10 drops of thymol blue indicator.
Add FB 4 from the burette until the end-point has been reached.
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 FB 4 added in each accurate titration.

(f)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i)

From your accurate titration results, obtain a suitable value for the volume of FB 4 to be used in your calculations.

$25.0\text{ cm}^3$ of FB 1 required ............................. $\text{cm}^3$ of FB 4.

(ii)

Calculate the number of moles of sodium hydroxide in the volume of FB 4 calculated in (i).

moles of $\text{NaOH}$ = ............................. mol

(iii)

Use your equation from (d) to calculate the number of moles of ethanedioic acid present in $25.0\text{ cm}^3$ of FB 1.

moles of $(\text{COOH})_2$ = ............................. mol

(g)

(i)

Use your answers to (c)(ii) and (f)(iii) to calculate the number of moles of sodium ethanedioate, $(\text{COONa})_2$ , present in $25.0\text{ cm}^3$ of FB 1.

moles of $(\text{COONa})_2$ = ............................. mol

(ii)

Calculate the mass of sodium ethanedioate present in $25.0\text{ cm}^3$ of FB 1.

mass of $(\text{COONa})_2$ = ............................. g

(iii)

Use your answer to (f)(iii) to calculate the mass of ethanedioic acid present in $25.0\text{ cm}^3$ of FB 1.

mass of $(\text{COOH})_2$ = ............................. g

(iv)

Calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FB 1.

percentage by mass of $(\text{COOH})_2$ = ............................. %

(h)

A student checked the formula of ethanedioic acid on the internet and found it to be $(\text{COOH})_2 \cdot 2\text{H}_2\text{O}$ . This differs from the formula $(\text{COOH})_2$ that you used in your calculations.

The FB 1 you used was made from $(\text{COOH})_2 \cdot 2\text{H}_2\text{O}$ and sodium ethanedioate.

State and explain the effect this knowledge has on;

(i)

the volume of FB 4 needed for reaction in (e),

(ii)

the calculated percentage by mass of $(\text{COOH})_2$ in the solid mixture used to prepare FB 1.

(i)

Another student suggested that the investigation could be improved by making the titrations more accurate. He said that the concentrations of FB 2 and FB 4 should be reduced.

State and explain whether or not this suggestion would make the titrations more accurate.

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