Chemistry 9701/33 — 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 · 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 concentration of hydrochloric acid, which supplies the $\text{H}^+$ ions in the following reaction.

\text{IO}_3^-(\text{aq}) + 5\text{I}^-(\text{aq}) + 6\text{H}^+(\text{aq}) \rightarrow 3\text{H}_2\text{O}(\text{l}) + 3\text{I}_2(\text{aq})

In the presence of an excess of $\text{IO}_3^-$ ions and an excess of $\text{I}^-$ ions, the amount of $\text{I}_2$ liberated is directly proportional to the amount of $\text{H}^+$ ions present and can be determined by titration with sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .

You are provided with the following reactants.

FA 1 hydrochloric acid
FA 2 containing $15.0\text{ g dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3\cdot 5\text{H}_2\text{O}$
aqueous potassium iodate(V), $\text{KIO}_3$
aqueous potassium iodide, $\text{KI}$

(a)

Method

Fill a burette with FA 2.
Pipette $25.0\text{ cm}^3$ of FA 1 into the conical flask.
Use a $25\text{ cm}^3$ measuring cylinder to add to the flask $10\text{ cm}^3$ of aqueous potassium iodate(V) and $10\text{ cm}^3$ of aqueous potassium iodide. There is an excess of each of these reagents.
Place the flask on a white tile.
Titrate the liberated iodine with FA 2.
During the titration the colour of the iodine in the solution will fade from red-brown to orange to yellow. The end-point occurs when the solution just goes colourless with the addition of a single drop of FA 2.
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 below all of your burette readings and the volume of FA 2 added in each accurate titration.
Make certain any recorded results show the precision of your practical work.

(b)

From your 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 FA 1 require .............. $\text{cm}^3$ of FA 2.

(c)

Calculations

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

(i)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of the sodium thiosulfate in FA 2.

FA 2 contains $15.0\text{ g dm}^{-3}\ \text{Na}_2\text{S}_2\text{O}_3\cdot 5\text{H}_2\text{O}$ .

[ $A_r$ : H, 1.0; O, 16.0; Na, 23.0; S, 32.1]

The concentration of sodium thiosulfate in FA 2 is ............................ $\text{mol dm}^{-3}$ .

(ii)

Calculate how many moles of $\text{Na}_2\text{S}_2\text{O}_3$ are contained in the volume of FA 2 recorded in (b).

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

(iii)

Calculate how many moles of iodine, $\text{I}_2$ reacted with the $\text{Na}_2\text{S}_2\text{O}_3$ in (ii).

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

............................ mol of iodine reacted with the sodium thiosulfate.

(iv)

Calculate how many moles of hydrochloric acid, $\text{HCl}$ , reacted with an excess of potassium iodate(V) and an excess of potassium iodide to produce the amount of iodine calculated in (iii).

\text{IO}_3^-(\text{aq}) + 5\text{I}^-(\text{aq}) + 6\text{H}^+(\text{aq}) \rightarrow 3\text{H}_2\text{O}(\text{l}) + 3\text{I}_2(\text{aq})

............................ mol of $\text{HCl}$ produced the amount of iodine calculated in (iii).

(v)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of $\text{HCl}$ in FA 1.

The concentration of $\text{HCl}$ in FA 1 is ....................... $\text{mol dm}^{-3}$ .

(d)

Each reading with a burette has a maximum error of $\pm 0.05\text{ cm}^3$ .
Grade B volumetric (bulb) pipettes are calibrated to $\pm 0.06\text{ cm}^3$ .

(i)

Calculate the maximum error in the volume run from the burette recorded in any titration.

The maximum error is ............................... $\text{cm}^3$ .

(ii)

Express the maximum error calculated in (i) as a percentage error for the volume calculated in (b).

The maximum error is .................................. %.

(iii)

Calculate the percentage error when $25.0\text{ cm}^3$ of FA 1 was pipetted into the conical flask.

The error was .................................. %.

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Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Qualitative Analysis10M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation15M

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