Chemistry 5070/32 — October/November 2013

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

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

Q117MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

P is an aqueous solution which contains a mixture of sodium carbonate, $\text{Na}_2\text{CO}_3$ , and sodium hydroxide, $\text{NaOH}$ .
The concentration of sodium carbonate in P is $0.0200\text{ mol / dm}^3$ .

You are to determine by titration the volume of dilute sulfuric acid, Q, needed to neutralise a volume of P, and then calculate the concentration of sodium hydroxide present.

Q is $0.100\text{ mol / dm}^3$ sulfuric acid, $\text{H}_2\text{SO}_4$ .

(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 P used was ______ $\text{cm}^3$ .

12M

(b)

Q is $0.100\text{ mol / dm}^3$ sulfuric acid, $\text{H}_2\text{SO}_4$ .
Calculate the number of moles of sulfuric acid present in your average volume of Q.

moles of sulfuric acid present = ______

(c)

The concentration of sodium carbonate, $\text{Na}_2\text{CO}_3$ , in P is $0.0200\text{ mol / dm}^3$ .
Calculate the number of moles of sodium carbonate present in your volume of P.

moles of sodium carbonate present = ______

(d)

Using your answer to (c), deduce the number of moles of sulfuric acid which react with the sodium carbonate present in your volume of P.

\text{Na}_2\text{CO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2

moles of sulfuric acid which react with the sodium carbonate = ______

(e)

Using your answers to (b) and (d), calculate the number of moles of sulfuric acid which react with the sodium hydroxide in your volume of P.

moles of sulfuric acid which react with the sodium hydroxide = ______

(f)

Using your answer to (e), calculate the concentration, in $\text{mol / dm}^3$ , of sodium hydroxide in P.

2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}

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

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Q2Qualitative Analysis · Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation23M

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