Chemistry 5070/32 — May/June 2013

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

Q116MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

A type of rust remover is an aqueous solution of phosphoric acid, $\text{H}_3\text{PO}_4$ .
$\mathbf{P}$ is a solution prepared by taking $100\text{ cm}^3$ of this rust remover and diluting the solution by adding distilled water until the total volume is $1.00\text{ dm}^3$ .

The amount of phosphoric acid present in solution $\mathbf{P}$ can be determined by titrating a volume of aqueous sodium hydroxide of known concentration with $\mathbf{P}$ , using an indicator.

Solution $\mathbf{Q}$ is $0.100\text{ mol / dm}^3$ sodium hydroxide.

(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 solution $\mathbf{Q}$ used was ______ $\text{cm}^3$ .

12M

(b)

$\mathbf{Q}$ is $0.100\text{ mol / dm}^3$ sodium hydroxide.

Using your results from (a), calculate the number of moles of phosphoric acid, $\text{H}_3\text{PO}_4$ , in $1.00\text{ dm}^3$ of $\mathbf{P}$ .

2\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_2\text{HPO}_4 + 2\text{H}_2\text{O}

moles of phosphoric acid in $1.00\text{ dm}^3$ of $\mathbf{P}$ = ______

(c)

Calculate the mass, in grams, of phosphoric acid present in $100\text{ cm}^3$ of the rust remover.
The relative formula mass of phosphoric acid is 98.

mass of phosphoric acid present in $100\text{ cm}^3$ of rust remover = ______ $\text{g}$

(d)

Given that $1\text{ cm}^3$ of the rust remover has a mass of $1.03\text{ g}$ , calculate the percentage by mass of phosphoric acid in the rust remover.

percentage by mass of phosphoric acid in the rust remover = ______

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