Chemistry 5070/42 — May/June 2022

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

questions

marks

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

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

A student determines the concentration of aqueous potassium manganate(VII), $\text{KMnO}_4$ , by titration with a solution of ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ .

An equation to represent this reaction is shown.

2\text{KMnO}_4 + 5\text{H}_2\text{C}_2\text{O}_4 + 6\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 10\text{CO}_2 + 8\text{H}_2\text{O} + 2\text{K}^+

(a)

Diagrams of some of the pieces of apparatus the student uses are shown.

Fig. 1.1

Name the three pieces of apparatus.

A = ______
B = ______
C = ______

(b)

The student:

records the mass of a sample of solid ethanedioic acid
dissolves the solid in distilled water and makes the solution up to $250\text{ cm}^3$
uses apparatus B to transfer $25.0\text{ cm}^3$ of the solution of $\text{H}_2\text{C}_2\text{O}_4$ into apparatus C
adds $10.0\text{ cm}^3$ of dilute sulfuric acid to apparatus C
fills apparatus A with the solution of $\text{KMnO}_4$
titrates the colourless solution of $\text{H}_2\text{C}_2\text{O}_4$ with the solution of $\text{KMnO}_4$ until the end-point is reached
repeats the titration three more times.

(i)

Suggest why sulfuric acid is added to apparatus C.

______

(ii)

State the colour change at the end-point.

from ______ to ______

(iii)

The diagrams show parts of apparatus A with the liquid levels at the beginning and the end of titration 1.

Fig. 1.2

Record the values in the results table.

Complete the results table.

titration	2	3	4
final reading / $\text{cm}^3$	24.8	25.9	24.9
initial reading / $\text{cm}^3$	0.0	0.8
volume used / $\text{cm}^3$	24.8		24.6
best titration results (✓)

(iv)

Tick (✓) the best titration results in the table.

Use the ticked values to calculate the average volume of $\text{KMnO}_4\text{(aq)}$ added in $\text{cm}^3$ .

average volume of $\text{KMnO}_4\text{(aq)}$ added = ______ $\text{cm}^3$

(c)

Another student repeats the experiment using the same method.

This student uses $1.08\text{ g}$ of ethanedioic acid to make up the $250\text{ cm}^3$ solution of $\text{H}_2\text{C}_2\text{O}_4$ .

The student obtains an average titration volume of $24.55\text{ cm}^3$ .

(i)

Calculate the number of moles of ethanedioic acid in $25.0\text{ cm}^3$ of the $\text{H}_2\text{C}_2\text{O}_4\text{(aq)}$ .

Show your working.

[ $M_r$ : $\text{H}_2\text{C}_2\text{O}_4$ , 90]

______ $\text{mol}$

(ii)

During the reaction, two moles of $\text{KMnO}_4$ react with five moles of $\text{H}_2\text{C}_2\text{O}_4$ .

Calculate the number of moles of $\text{KMnO}_4$ in $24.55\text{ cm}^3$ of the aqueous potassium manganate(VII).

______ $\text{mol}$

(iii)

Calculate the concentration, in $\text{mol / dm}^3$ , of the $\text{KMnO}_4\text{(aq)}$ .

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

(iv)

Calculate the concentration, in $\text{g / dm}^3$ , of the $\text{KMnO}_4\text{(aq)}$ .

Give your answer to three significant figures.

[ $M_r$ : $\text{KMnO}_4$ , 158]

______ $\text{g / dm}^3$

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