Chemistry 5070/32 — May/June 2016

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 · Experimental Contexts · Qualitative Analysis

Q118MExperimental ContextsObservations 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.

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.
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 P with Q.

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

(a)

Put Q into the burette.

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

Summary

Tick (✓) the best titration results.

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

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

12M

(b)

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 P. Give your answer to three significant figures.

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

(c)

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

concentration of sodium carbonate in 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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