Chemistry 5070/31 — May/June 2011

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

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Topics Experimental Contexts · Observations and Measurements · Use of Techniques, Apparatus and Materials · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q116MExperimental ContextsObservations and MeasurementsUse of Techniques, Apparatus and MaterialsAnalysis, Conclusions and EvaluationFree sample

Seaweed can be used as a commercial source of iodine. The amount of iodine present in a sample of seaweed is often stated in parts per million, ppm. For instance, if a sample contains 200 ppm, then there are 200 g of iodine in every 1 000 000 g of seaweed.

You are provided with an aqueous solution of iodine which has been obtained from seaweed. You are required to determine its concentration by titration with sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ , using starch as an indicator and then calculate how much iodine is present in the seaweed.

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

$\mathbf{P}$ is the aqueous solution of iodine.
$\mathbf{Q}$ is 0.100 $\text{mol / dm}^3$ sodium thiosulfate.

(a)

Put $\mathbf{Q}$ into the burette.

Pipette a 25.0 $\text{cm}^3$ (or 20.0 $\text{cm}^3$ ) portion of $\mathbf{P}$ into a flask.

Add $\mathbf{Q}$ from the burette until the red-brown colour fades to pale yellow, then add a few drops of the starch indicator. This will give a dark blue solution. Continue adding $\mathbf{Q}$ slowly from the burette until one drop of $\mathbf{Q}$ causes the blue colour to disappear, leaving a colourless solution.

Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results.

Results

Burette readings

titration number	1	2
final reading / $\text{cm}^3$
initial reading / $\text{cm}^3$
volume of $\mathbf{Q}$ used / $\text{cm}^3$
best titration results (✓)

Summary

Tick (✓) the best titration results.

Using these results, the average volume of $\mathbf{Q}$ required was ______ $\text{cm}^3$ .

Volume of solution $\mathbf{P}$ used was ______ $\text{cm}^3$ .

12M

(b)

$\mathbf{Q}$ is 0.100 $\text{mol / dm}^3$ sodium thiosulfate.

Using your results from (a), calculate the concentration, in $\text{mol / dm}^3$ , of iodine in $\mathbf{P}$ .

concentration of iodine in $\mathbf{P}$ = ______ $\text{mol / dm}^3$

(c)

Using your answer from (b), calculate the mass, in g, of iodine in 1 $\text{dm}^3$ of $\mathbf{P}$ .
[The relative atomic mass of iodine is 127.]

mass of iodine in 1 $\text{dm}^3$ of $\mathbf{P}$ = ______ $\text{g}$

(d)

If all the iodine present in 1 $\text{dm}^3$ of $\mathbf{P}$ was obtained from 15 000 g of seaweed, calculate the amount, in ppm, of iodine present in the seaweed.

amount of iodine present in the seaweed = ______ ppm

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