Chemistry 5070/42 — May/June 2020

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

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

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

Iron tablets are used to treat iron deficiency in the body.

Iron tablets contain iron(II) ions, $\text{Fe}^{2+}$ .

A student does a series of titrations with aqueous potassium manganate(VII), $\text{KMnO}_4$ , to determine the percentage of iron in some iron tablets.

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

Fig. 1.1

(a)

Name the three pieces of apparatus.

A ______

B ______

C ______

(b)

The student:

records the total mass of five iron tablets
crushes the tablets, dissolves them in distilled water and makes the solution up to $250\text{ cm}^3$
uses apparatus C to transfer $25.0\text{ cm}^3$ of the solution of $\text{Fe}^{2+}$ ions into a conical flask
uses apparatus B to add $10.0\text{ cm}^3$ of dilute sulfuric acid to the conical flask
fills apparatus A with $0.00500\text{ mol / dm}^3$ $\text{KMnO}_4\text{(aq)}$
titrates the solution of $\text{Fe}^{2+}$ with the $0.00500\text{ mol / dm}^3$ $\text{KMnO}_4\text{(aq)}$ until the first permanent pink colour is seen in the conical flask
repeats the titration three times.

The equation for the reaction is shown.

\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}

(i)

Suggest why dilute sulfuric acid is added to the conical flask.

______

(ii)

Give the formula of the ion responsible for the pink colour seen at the end-point.

______

(iii)

The diagrams show parts of apparatus A 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 / $\text{cm}^3$	17.2	34.1	16.9
initial reading / $\text{cm}^3$	0.0	17.2
volume used / $\text{cm}^3$		16.9	16.7
best titration results (✓)

(iv)

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

Use the ticked values to calculate the average volume of $0.00500\text{ mol / dm}^3$ $\text{KMnO}_4\text{(aq)}$ used.

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

(c)

A second student does another series of titrations using the same method and $0.00500\text{ mol / dm}^3$ $\text{KMnO}_4\text{(aq)}$ .

This student obtains an average volume of $16.9\text{ cm}^3$ .

The equation for the reaction is shown.

\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}

(i)

Calculate the number of moles of $\text{MnO}_4^-$ used by the second student.

______ $\text{mol}$

(ii)

Calculate the number of moles of $\text{Fe}^{2+}$ ions present in the $25.0\text{ cm}^3$ sample of solution.

______ $\text{mol}$

(iii)

Calculate the total mass of $\text{Fe}^{2+}$ ions in the five tablets.

[ $A_r$ : Fe, 56]

______ $\text{g}$

(iv)

The total mass of the five tablets is $1.83\text{ g}$ .

Calculate the percentage, by mass, of iron in the tablets.

Give your answer to three significant figures.

______ %

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