Chemistry 9701/34 — May/June 2012

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 ObservationsAnalysis, Conclusions and EvaluationFree sample

You are to determine the percentage by mass of sodium ethanedioate in a mixture of sodium ethanedioate and ethanedioic acid.

This experiment involves two steps.
In step one, you will carry out a titration to find the amount of acid, $\text{C}_2\text{O}_4\text{H}_2$ , present in FB 3.
In step two, you will carry out a second titration to find the total amount of ethanedioate ion, $\text{C}_2\text{O}_4^{2-}$ , present in FB 3.
Finally, you will use the values found in the two steps to calculate the percentage by mass of sodium ethanedioate in FB 3.

FB 1 is $0.100\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
FB 2 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FB 3 is a mixture of aqueous sodium ethanedioate, $\text{C}_2\text{O}_4\text{Na}_2$ , and ethanedioic acid, $\text{C}_2\text{O}_4\text{H}_2$ .
FB 4 is approximately $2\text{ mol dm}^{-3}$ sulfuric acid.
phenolphthalein indicator

Read through the whole method before starting any practical work.

(a)

Method

Step 1

Fill the burette labelled FB 1 with FB 1.
Pipette $25.0\text{ cm}^3$ of FB 3 into a conical flask.
Add about three drops of phenolphthalein.
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 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 FB 1 added in each accurate titration.

Step 2

Pipette $25.0\text{ cm}^3$ of FB 3 into a conical flask.
Using a measuring cylinder, add about $25\text{ cm}^3$ of $2\text{ mol dm}^{-3}$ sulfuric acid, FB 4, to the flask.
Place the conical flask on a tripod and gauze and heat to about $80\text{ }^{\circ}\text{C}$ .
Fill the burette labelled FB 2 with FB 2.
Use an appropriate method to carefully transfer the hot conical flask onto a white tile under the burette.
Titrate the mixture in the conical flask with FB 2 until a permanent pale pink colour is seen. If a permanent brown colour is seen, stop the titration and begin Step 2 again.
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 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 FB 2 added in each accurate titration.

12M

(b)

Calculations

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

(i)

From your accurate titration results in Step 1, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

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

(ii)

Write an equation for the reaction between sodium hydroxide and ethanedioic acid to give sodium ethanedioate and water.

(iii)

Use your answer from (i) to calculate the number of moles of sodium hydroxide, FB 1, required to react with $25.0\text{ cm}^3$ of FB 3 in Step 1.

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

(iv)

Use your answer to (iii) to determine the number of moles of ethanedioic acid in $25.0\text{ cm}^3$ of FB 3.

moles of $\text{C}_2\text{O}_4\text{H}_2$ in $25.0\text{ cm}^3$ of FB 3 = ...................... mol

(c)

(i)

From your accurate titration results in Step 2, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

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

(ii)

Use your answer from (i) to calculate the number of moles of potassium manganate(VII), FB 2, required to react with $25.0\text{ cm}^3$ of FB 3 in Step 2.

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

(iii)

The equation for the reaction between acidified manganate(VII) ions and ethanedioate ions is shown below.

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

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

total moles of $\text{C}_2\text{O}_4^{2-}$ in $25.0\text{ cm}^3$ of FB 3 = ...................... mol

(iv)

Use your answers to (b)(iv) and (c)(iii) to calculate the number of moles of ethanedioate ions which came from the sodium ethanedioate dissolved in $25.0\text{ cm}^3$ of FB 3.

moles of $\text{C}_2\text{O}_4^{2-}$ from $\text{C}_2\text{O}_4\text{Na}_2$ in $25.0\text{ cm}^3$ of FB 3 = ...................... mol

(d)

(i)

Use your answer to (b)(iv) to calculate the mass of ethanedioic acid, $\text{C}_2\text{O}_4\text{H}_2$ , in $25.0\text{ cm}^3$ of FB 3.

[ $A_r$ : H, 1.0; C, 12.0; O, 16.0]

(If you were unable to answer (b)(iv), you may assume that the number of moles of ethanedioic acid is $6.51 \times 10^{-4}\text{ mol}$ .)

mass of ethanedioic acid = ...................... g

(ii)

Use your answer to (c)(iv) to calculate the mass of sodium ethanedioate, $\text{C}_2\text{O}_4\text{Na}_2$ , in $25.0\text{ cm}^3$ of FB 3.

[ $A_r$ : C, 12.0; O, 16.0; Na, 23.0]

(If you were unable to answer (c)(iv), you may assume that the number of moles of sodium ethanedioate is $4.13 \times 10^{-4}\text{ mol}$ .)

mass of sodium ethanedioate = ...................... g

(iii)

Calculate the percentage by mass of sodium ethanedioate present in FB 3.

Percentage by mass of sodium ethanedioate present is ...................... %.

(e)

(i)

What is the maximum error in a single burette reading?

maximum error = ...................... $\text{cm}^3$

(ii)

A student suggested that using a burette to measure the $25.0\text{ cm}^3$ of acid would give a more accurate result than using a pipette. The percentage error of a $25.0\text{ cm}^3$ pipette is $0.24\%$ . Is the student correct? Explain your answer.

(f)

A student decided to use a $25.0\text{ cm}^3$ pipette instead of a measuring cylinder to measure the volume of FB 4 in Step 2.

State and explain whether this alteration will improve the accuracy of the calculation of the percentage by mass of sodium ethanedioate in the mixture.

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Q2Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations15M

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