Chemistry 5070/31 — May/June 2016

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 · Use of Techniques, Apparatus and Materials · Observations and Measurements · Qualitative Analysis

Q118MExperimental ContextsUse of Techniques, Apparatus and MaterialsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

Sodium percarbonate is often used to remove stains from clothing. It is a white solid with the formula $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}_2$ . When the solid dissolves in water, it produces a solution containing sodium carbonate and hydrogen peroxide.

$\mathbf{P}$ is a solution containing $7.85\text{ g}$ of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}_2$ in $1.00\text{ dm}^3$ of solution.
$\mathbf{Q}$ is $0.100\text{ mol / dm}^3$ hydrochloric acid.

You are to determine the value of $x$ in the formula $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}_2$ by titrating the sodium carbonate in $\mathbf{P}$ with $\mathbf{Q}$ .

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

(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 and titrate with $\mathbf{Q}$ , 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 $\mathbf{Q}$ 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 $0.100\text{ mol / dm}^3$ hydrochloric acid.
Using your results from (a), calculate the concentration, in $\text{mol / dm}^3$ , of sodium carbonate in $\mathbf{P}$ . Give your answer to three significant figures.

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

(c)

Using your answer from (b), calculate the concentration, in $\text{g / dm}^3$ , of sodium carbonate in $\mathbf{P}$ .
[The relative formula mass of sodium carbonate is 106.]

concentration of sodium carbonate in $\mathbf{P}$ = ______ $\text{g / dm}^3$

(d)

Using your answer from (c), calculate the mass of hydrogen peroxide in $7.85\text{ g}$ of sodium percarbonate.

mass of hydrogen peroxide = ______ $\text{g}$

(e)

Calculate the value of $x$ in the formula $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}_2$ .
[The relative formula mass of $\text{H}_2\text{O}_2$ is 34.]

The value of $x$ is = ______

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