Chemistry 5070/32 — October/November 2010

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

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Topics Experimental Contexts · Analysis, Conclusions and Evaluation · Observations and Measurements · Qualitative Analysis

Q117MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

A large volume of water has been contaminated with acid. Before the contaminated water can be disposed of, the acid must be neutralised by adding calcium carbonate.

\text{CaCO}_3 + 2\text{H}^+ \rightarrow \text{Ca}^{2+} + \text{H}_2\text{O} + \text{CO}_2

You are to determine by titration the concentration of hydrogen ions present in the contaminated water and then calculate the mass of calcium carbonate needed to neutralise all the acid.

Solution $\mathbf{P}$ is a sample of the contaminated water.
Solution $\mathbf{Q}$ is $0.100\text{ mol/dm}^3$ sodium hydroxide.

(a)

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

Pipette a $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$ ) portion of $\mathbf{Q}$ into a flask and titrate with $\mathbf{P}$ , 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 $\mathbf{P}$ used / $\text{cm}^3$
best titration results (✓)

Summary

Tick (✓) the best titration results.

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

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

12M

(b)

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

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

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

(c)

The volume of the contaminated water is $10\,000\text{ dm}^3$ .

Calculate the number of moles of hydrogen ions in this volume of contaminated water.

moles of hydrogen ions in $10\,000\text{ dm}^3$ of $\mathbf{P}$ = ______

(d)

Using your answer from (c), calculate the minimum mass of calcium carbonate needed to neutralise all the acid in $10\,000\text{ dm}^3$ of the contaminated water.

[The relative formula mass of calcium carbonate is 100.]

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

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