Chemistry 5070/31 — October/November 2017

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

Q117MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

The composition of concentrated nitric acid can be determined by diluting a sample of the acid and then titrating aqueous sodium carbonate of known concentration with the diluted acid.

P is dilute nitric acid. It has been prepared by adding $10.0\text{ cm}^3$ of the concentrated nitric acid to distilled water and then making the total volume of the solution up to $500\text{ cm}^3$ in a volumetric flask by adding distilled water.

Q is $0.153\text{ mol / dm}^3$ 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 the best titration results, the average volume of P required was ______ $\text{cm}^3$ .

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

12M

(b)

Q is $0.153\text{ mol / dm}^3$ sodium carbonate.

Using your results from (a), calculate the concentration, in $\text{mol / dm}^3$ , of nitric acid in P. Give your answer to three significant figures.

\text{Na}_2\text{CO}_3 + 2\text{HNO}_3 \rightarrow 2\text{NaNO}_3 + \text{H}_2\text{O} + \text{CO}_2

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

(c)

Using your answer from (b), calculate the number of moles of nitric acid in $10.0\text{ cm}^3$ of concentrated nitric acid.

moles of nitric acid in $10.0\text{ cm}^3$ of concentrated nitric acid = ______

(d)

Using your answer from (c), calculate the concentration, in $\text{mol / dm}^3$ , of concentrated nitric acid.

concentration of concentrated nitric acid = ______ $\text{mol / dm}^3$

(e)

Using your answer from (d), calculate the mass, in $\text{g}$ , of nitric acid in $1\text{ dm}^3$ of concentrated nitric acid.

[The relative formula mass of nitric acid is 63.]

mass of nitric acid in $1\text{ dm}^3$ of concentrated nitric acid = ______ $\text{g}$

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

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