Chemistry 5070/31 — May/June 2023

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

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Topics Experimental Contexts · Observations and Measurements · Planning Experiments and Investigations · Use of Techniques, Apparatus and Materials · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q117MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationPlanning Experiments and InvestigationsUse of Techniques, Apparatus and MaterialsFree sample

White vinegar is a colourless solution containing ethanoic acid.

You are provided with two samples of white vinegar labelled A and B.

You are going to investigate the reaction between these samples and $0.400\text{ mol / dm}^3$ aqueous sodium hydroxide.

Read all the instructions carefully before starting the experiments.

Instructions

You are going to do four titration experiments.

Rinse and fill a burette with A.

(a)

Experiment 1

Use a volumetric pipette to add $25.0\text{ cm}^3$ of $0.400\text{ mol / dm}^3$ aqueous sodium hydroxide to a conical flask.
Add five drops of thymolphthalein indicator to the conical flask.
Record the initial burette reading in Table 1.1.
Add A from the burette while swirling the flask, adding drop by drop near the end-point, until the solution just changes colour.
Record the final burette reading in Table 1.1.

Experiment 2

Empty the conical flask and rinse it with distilled water.
Refill the burette if necessary.
Repeat Experiment 1.

Complete Table 1.1 with the volume used in each experiment.

Table 1.1

	Experiment 1	Experiment 2
final burette reading / $\text{cm}^3$
initial burette reading / $\text{cm}^3$
volume of A used / $\text{cm}^3$

(b)

Experiments 3 and 4

Pour away any A remaining in the burette.
Rinse and fill the burette with B.
Empty the conical flask and rinse it with distilled water.
Repeat Experiments 1 and 2 using B.
Record the initial and final burette readings for Experiments 3 and 4 in Table 1.2.
Complete Table 1.2 with the volumes used in Experiments 3 and 4.

Table 1.2

	Experiment 3	Experiment 4
final burette reading / $\text{cm}^3$
initial burette reading / $\text{cm}^3$
volume of B used / $\text{cm}^3$

(c)

State the colour change observed in the flask in each experiment.

from ______ to ______

(d)

Calculate the mean volume of A and B needed to neutralise $25.0\text{ cm}^3$ of $0.400\text{ mol / dm}^3$ aqueous sodium hydroxide.

mean volume of A = ______ $\text{cm}^3$
mean volume of B = ______ $\text{cm}^3$

(e)

Suggest why the titrations using A and B are repeated.

______

(f)

The equation for the reaction between ethanoic acid and sodium hydroxide is shown.

\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}

The answer to (d) shows the mean volume of B used to neutralise $25.0\text{ cm}^3$ of $0.400\text{ mol / dm}^3$ aqueous sodium hydroxide.

Calculate the concentration of ethanoic acid in B.

Give your answer to an appropriate number of significant figures.

concentration = ______ $\text{mol / dm}^3$

(g)

Use your answer to (f) to calculate the mass of ethanoic acid in $500\text{ cm}^3$ of B.

[ $M_r$ : ethanoic acid, 60]

mass = ______ $\text{g}$

(h)

Use your answers to (d) and (g) to calculate the mass of ethanoic acid in $500\text{ cm}^3$ of A.

mass = ______ $\text{g}$

(i)

Suggest why the conical flask should be rinsed with distilled water and not $0.400\text{ mol / dm}^3$ aqueous sodium hydroxide between titrations.

______

(j)

Some vinegars are brown rather than colourless.

Suggest why this titration method would not be suitable for finding the concentration of ethanoic acid in brown vinegar.

______

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2 more questions

Q2Qualitative Analysis · Experimental Contexts · Observations and Measurements17M
Q3Planning Experiments and Investigations · Experimental Contexts · Use of Techniques, Apparatus and Materials6M

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