Edexcel A-Level Chemistry AS 3 P4 Evaluating Results and Conclusions Questions

Practise Edexcel IAL Chemistry 3.P4 by tracing errors and procedural effects, judging conclusions against trends and deciding when anomalous results require repeats.

Exam points

evaluate validity by tracing impurities, losses and procedural effects
use trends and evidence to judge whether a conclusion is supported
identify anomalies and decide whether experiments should be repeated

[Maximum number: 1]

Precipitation reactions can be used to determine the formulae of compounds.
A bottle has a torn label which shows that it contains a nitrate with the formula X(NO_3)_2.

Figure for Question Edexcel A-Level Chemistry AS 3 P4 Evaluating Results and Conclusions Questions question 1 — Edexcel A-Level Chemistry AS

Procedure
- a technician dissolved a sample of 12.41 g of the nitrate in deionised water to make 100 cm^3 of solution
- the technician pipetted 5.0 cm^3 of 1.50 mol dm^-3 potassium iodide into each of a series of test tubes
- each test tube then had a volume of the nitrate solution added to it as shown in the table
- a cloudy yellow solution formed, and the precipitate was allowed to settle
- the height of the precipitate was then measured.

The equation for the reaction is

\mathrm{X}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI}(\mathrm{aq}) \rightarrow \mathrm{XI}_{2}(\mathrm{~s})+2 \mathrm{KNO}_{3}(\mathrm{aq})

The results of the experiment are shown.

Table for Question Edexcel A-Level Chemistry AS 3 P4 Evaluating Results and Conclusions Questions question 1 — Edexcel A-Level Chemistry AS

Suggest why some of the precipitate heights may be above the maximum height expected. Assume there were no measurement errors.

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Edexcel A-Level Chemistry AS 3 P4 Evaluating Results and Conclusions Questions question 1