Chemistry 9701/36 — October/November 2010

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 ObservationAnalysis, Conclusions and EvaluationPresentation of Data and ObservationsFree sample

You must prepare Flask A and Flask B in Question 2 before starting Question 1. Shake each flask periodically during the time you spend on Question 1.

FB 1 is $0.125\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .
FB 2 is an aqueous solution of sodium hydroxide, $\text{NaOH}$ .

You are to determine the concentration, in $\text{mol dm}^{-3}$ , of the sodium hydroxide in FB 2.

(a)

Method

Fill a burette with FB 1.
Run between $45.50\text{ cm}^3$ and $46.50\text{ cm}^3$ of FB 1 from the burette into the $250\text{ cm}^3$ graduated (volumetric) flask, labelled FB 3.
Make up to the mark with distilled water.
Shake the flask to mix the solution.

In the space below record your burette readings and the volume of FB 1 added to the graduated flask.

You are reminded to shake Flask A and Flask B periodically.

Titration

Fill a second burette with FB 2.
Pipette $25.0\text{ cm}^3$ of FB 3, the diluted acid, into a conical flask.
Add to the flask a few drops of phenolphthalein indicator.
Place the flask on a white tile.
Titrate the acid in the flask with FB 2.
At the end-point a "permanent" pink colour will remain in the solution.
Note: The "permanent" pink colour will fade over several minutes as carbon dioxide is absorbed from the atmosphere.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

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

Carry out as many accurate titrations as you think necessary to obtain consistent results
Record in a suitable form on page 3 all of your burette readings and the volume of FB 2 added in each accurate titration.
Make certain any recorded results show the precision of your practical work.

You will require the burette containing FB 2 for Question 2.

(b)

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

$25.0\text{ cm}^3$ of FB 3 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 how many moles of $\text{H}_2\text{SO}_4$ in FB 1 were run from the burette into the $250\text{ cm}^3$ graduated, (volumetric) flask.

............................ mol of $\text{H}_2\text{SO}_4$ were run from the burette into the graduated flask.

(ii)

Calculate how many moles of $\text{H}_2\text{SO}_4$ in FB 3 were pipetted from the graduated flask into the conical flask in each titration.

............................ mol of $\text{H}_2\text{SO}_4$ were pipetted into the conical flask.

(iii)

Calculate how many moles of $\text{NaOH}$ reacted with the $\text{H}_2\text{SO}_4$ in (ii).

\text{H}_2\text{SO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}

The $\text{H}_2\text{SO}_4$ in the titration flask reacted with ............................ mol of $\text{NaOH}$ .

(iv)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of $\text{NaOH}$ in FB 2.

The concentration of $\text{NaOH}$ in FB 2 is ............................ $\text{mol dm}^{-3}$ .

(d)

The individual error in any burette reading is $\pm 0.05\text{ cm}^3$ .
Two students, A and B, record identical burette readings.

\begin{aligned} \text{final burette reading} && 25.60\text{ cm}^3 \\ \text{initial burette reading} && 1.35\text{ cm}^3 \\ \text{volume added} && 24.25\text{ cm}^3 \end{aligned}

Explain the following.

(i)

The initial burette reading made by student A was $0.05\text{ cm}^3$ greater than the true value but the volume added was exactly $24.25\text{ cm}^3$ .

(ii)

The initial burette reading made by student B was $0.05\text{ cm}^3$ less than the true value and the actual volume added was exactly $24.15\text{ cm}^3$ .

(e)

In the instructions for the experiment you were told that the "permanent" pink colour at the end-point would fade over a few minutes as carbon dioxide is absorbed from the atmosphere.

(i)

Explain why absorption of carbon dioxide at the end-point would reverse the indicator colour change seen in the titration.

(ii)

Suggest a modification to the titration method, using the same indicator, that would overcome this problem.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation8M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation · Presentation of Data and Observations15M

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