Chemistry 5070/32 — May/June 2023

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

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marks

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

Q117MExperimental ContextsObservations and MeasurementsUse of Techniques, Apparatus and MaterialsAnalysis, Conclusions and EvaluationFree sample

Aqueous ammonia is a solution of ammonia gas.

You are provided with two samples of aqueous ammonia labelled A and B.

You are going to investigate the reaction between these samples and $0.500\text{ mol / dm}^3$ hydrochloric acid.

Read all the instructions carefully before starting the experiments.

Instructions

You are going to do four titration experiments.

Rinse and fill a burette with $0.500\text{ mol / dm}^3$ hydrochloric acid.

(a)

Experiment 1

Use a volumetric pipette to add $25.0\text{ cm}^3$ of A to a conical flask.
Add five drops of methyl orange indicator to the conical flask.
Record the initial burette reading in Table 1.1.
Add $0.500\text{ mol / dm}^3$ hydrochloric acid 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 used / $\text{cm}^3$

(b)

Experiments 3 and 4

Empty the conical flask and rinse it with distilled water.
Refill the burette if necessary.
Repeat Experiments 1 and 2 using B instead of A.
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 used / $\text{cm}^3$

(c)

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

from ______ to ______

(d)

Suggest why you are provided with a white tile for use in these titrations.

______

(e)

Calculate the mean volume of $0.500\text{ mol / dm}^3$ hydrochloric acid needed to neutralise $25.0\text{ cm}^3$ of A and of B.

mean volume of acid needed for solution A = ______ $\text{cm}^3$
mean volume of acid needed for solution B = ______ $\text{cm}^3$

(f)

The equation for the reaction between hydrochloric acid and aqueous ammonia is shown.

\text{HCl} + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl}

Use the mean volume of acid needed to neutralise $25.0\text{ cm}^3$ of A from (e) to calculate the concentration of ammonia in A.

Give your answer to an appropriate number of significant figures.

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

(g)

Calculate the volume of ammonia gas measured at room temperature and pressure, r.t.p., dissolved in $25.0\text{ cm}^3$ of A.

The volume of one mole of any gas at r.t.p. is $24\text{ dm}^3$ .

volume = ______ $\text{dm}^3$

(h)

Use your answers to (e) and (g) to calculate the volume of ammonia gas measured at r.t.p. dissolved in $25.0\text{ cm}^3$ of B.

volume = ______ $\text{dm}^3$

(i)

The burette is rinsed with $0.500\text{ mol / dm}^3$ hydrochloric acid immediately before it is filled.

Explain why the burette should not be rinsed with distilled water immediately before it is filled with $0.500\text{ mol / dm}^3$ hydrochloric acid.

______

(j)

Suggest why universal indicator is not suitable for use in these titrations.

______

The rest of this paper

2 more questions

Q2Qualitative Analysis · Experimental Contexts17M
Q3Planning Experiments and Investigations6M

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