Chemistry 5070/42 — May/June 2019

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

questions

marks

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

Q114MUse of Techniques, Apparatus and MaterialsExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

Milk of Magnesia is a liquid medicine used to treat indigestion.

Milk of Magnesia contains magnesium hydroxide, $\text{Mg(OH)}_2$ .

A student does a series of titrations to determine the percentage by mass of magnesium hydroxide in Milk of Magnesia.

Diagrams of some of the apparatus used by the student are shown.

Fig. 1.1

(a)

Name the three pieces of apparatus.

A ______

B ______

C ______

(b)

The student:

puts $5.0\text{ cm}^3$ of the Milk of Magnesia into apparatus A and adds some distilled water
adds a few drops of a suitable indicator to the mixture in apparatus A
fills apparatus C with $0.600\text{ mol / dm}^3$ hydrochloric acid
titrates the contents of apparatus A with the $0.600\text{ mol / dm}^3$ hydrochloric acid until the indicator changes colour
records the readings.

The student repeats the procedure three further times.

(i)

The diagrams show parts of apparatus C with the liquid levels at the beginning and the end of titration 3.

Fig. 1.2

Record the values in the results table.

Complete the results table for each of titrations 1, 3 and 4.

titration number	1	2	4
final reading / $\text{cm}^3$	24.1	47.5	24.4
initial reading / $\text{cm}^3$	0.0	23.7
volume of $0.600\text{ mol / dm}^3$ hydrochloric acid used / $\text{cm}^3$		23.8	23.6
best titration results ( $\checkmark$ )

(ii)

Tick ( $\checkmark$ ) the best titration results in the table.

Use these best titration results to calculate the average volume of $0.600\text{ mol / dm}^3$ hydrochloric acid used.

average volume = ______ $\text{cm}^3$

(iii)

Suggest why the volume of distilled water the student adds to apparatus A does not affect the results of the titrations.

______

(c)

A second student does another series of titrations using the same method. This student obtains an average volume of $23.3\text{ cm}^3$ .

The equation for the reaction during the titration is shown.

\text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}

(i)

Calculate the number of moles of $0.600\text{ mol / dm}^3$ hydrochloric acid used by this second student.

______ moles

(ii)

Calculate the number of moles of magnesium hydroxide present in $5.0\text{ cm}^3$ of the Milk of Magnesia.

______ moles

(iii)

Calculate the mass of magnesium hydroxide in $5.0\text{ cm}^3$ of the Milk of Magnesia.
[ $A_r$ : $\text{Mg}, 24$ ; $\text{O}, 16$ ; $\text{H}, 1$ ]

______ $\text{g}$

(iv)

The density of the Milk of Magnesia is $2.34\text{ g / cm}^3$ .

\text{density in g / cm}^3 = \frac{\text{mass in g}}{\text{volume in cm}^3}

Calculate the mass of $5.0\text{ cm}^3$ of the Milk of Magnesia.

______ $\text{g}$

(v)

Calculate the percentage by mass of magnesium hydroxide in the Milk of Magnesia.

______ %

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