Chemistry 5070/32 — October/November 2011

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

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

Q116MObservations and MeasurementsUse of Techniques, Apparatus and MaterialsAnalysis, Conclusions and EvaluationFree sample

Solid calcium carbonate is sometimes found on indoor surfaces which are in contact with water. This solid is called scale. Hydrochloric acid can be used as a scale-remover. It removes the scale by reacting with the carbonate.

You are to determine the concentration of the acid in the scale-remover by titrating a diluted solution of the acid with aqueous sodium carbonate.

P is dilute hydrochloric acid. It has been made by adding distilled water to $100\text{ cm}^3$ of scale-remover until the volume was $1\,000\text{ cm}^3$ .

Q is $0.0500\text{ mol/dm}^3$ sodium carbonate.

(a)

Put P into the burette.

Pipette a $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$ ) portion of Q into a flask and titrate with P, using the indicator provided.

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 P used / $\text{cm}^3$
best titration results (✓)

Summary

Tick (✓) the best titration results.

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

Volume of Q used was ______ $\text{cm}^3$ .

12M

(b)

Q is $0.0500\text{ mol/dm}^3$ sodium carbonate.

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

\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2

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

(c)

Using your answer from (b) and information given in the question, calculate the concentration of hydrochloric acid in the scale-remover.

concentration of hydrochloric acid in scale-remover = ______ $\text{mol/dm}^3$

(d)

A bottle of the scale-remover contains $2\,000\text{ cm}^3$ of the hydrochloric acid solution. Using your answer from (c), calculate the maximum mass of calcium carbonate that can be removed by treatment with a bottle of the scale-remover.

The relative formula mass of calcium carbonate is 100.

\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

mass of calcium carbonate removed = ______ $\text{g}$

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Q2Experimental Contexts · Qualitative Analysis · Observations and Measurements · Analysis, Conclusions and Evaluation24M

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