Chemistry 5070/32 — May/June 2012

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

Q118MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

Ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ , forms crystals which are hydrated and have the formula $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ .

$\mathbf{P}$ is a solution containing $9.45\text{ g}$ of $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ in $1.00\text{ dm}^3$ of solution.
$\mathbf{Q}$ is $0.150\text{ mol / dm}^3$ sodium hydroxide, $\text{NaOH}$ .

You are to determine the value of $x$ in the formula $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ by titrating $\mathbf{Q}$ with $\mathbf{P}$ .

(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 $0.150\text{ mol / dm}^3$ sodium hydroxide.
Using your results from (a), calculate the concentration, in $\text{mol / dm}^3$ , of the ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ , in $\mathbf{P}$ .

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

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

(c)

Using your answer from (b), calculate the concentration, in $\text{g / dm}^3$ , of ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ , in $\mathbf{P}$ .
[The relative formula mass of ethanedioic acid is 90.]

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

(d)

Using your answer from (c), calculate the mass of water in $9.45\text{ g}$ of hydrated ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ .

mass of water = ______ $\text{g}$

(e)

Calculate the value of $x$ in the formula $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ .
[The relative formula mass of $\text{H}_2\text{O}$ is 18.]

value of $x$ = ______

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Q2Experimental Contexts · Qualitative Analysis · Observations and Measurements · Analysis, Conclusions and Evaluation22M

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