Chemistry 5070/32 — October/November 2016

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

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Topics Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q117MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

The percentage by mass of iron in steel can be determined by converting all the iron in the steel into aqueous iron(II) ions and titrating portions of the solution with aqueous potassium manganate(VII).

No indicator is needed for this titration as the products of the reaction are almost colourless and one drop of aqueous potassium manganate(VII) in excess produces a permanent pale pink colour.

P is an aqueous solution of iron(II) ions, $\text{Fe}^{2+}$ . The solution was prepared by converting all the iron in a $3.12\text{ g}$ sample of steel into iron(II) ions and making the final volume up to $500\text{ cm}^3$ by adding water.

Q is $0.0200\text{ mol / dm}^3$ potassium manganate(VII).

(a)

Put Q into the burette.

The colour of Q makes it difficult to see the bottom of the meniscus so you should take all your readings using the top of the meniscus.

Pipette a $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$ ) portion of P into a flask.

Add Q from the burette. At first the purple colour disappears quickly but as more Q is added the colour disappears less quickly. At the end-point, one drop of Q produces a pale pink colour that does not disappear on swirling.

Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results.

Results

Burette readings

titration number	1	2
final reading / $\text{cm}^3$
initial reading / $\text{cm}^3$
volume of Q used / $\text{cm}^3$
best titration results (✓)

Summary

Tick (✓) the best titration results.

Using these results, the average volume of Q required was ______ $\text{cm}^3$ .

Volume of P used was ______ $\text{cm}^3$ .

12M

(b)

Q is $0.0200\text{ mol / dm}^3$ potassium manganate(VII).

Calculate the number of moles of potassium manganate(VII) present in the average volume of Q required.

number of moles of potassium manganate(VII) = ______

(c)

Using your answer from (b), calculate the number of moles of iron(II) ions present in the volume of P used in each titration.
[Five moles of iron(II) ions react with one mole of potassium manganate(VII) ions].

number of moles of iron(II) ions = ______

(d)

Using your answer from (c), calculate the number of moles of iron(II) ions in $500\text{ cm}^3$ of P.

number of moles of iron(II) ions in $500\text{ cm}^3$ of P = ______

(e)

Using your answer from (d), calculate the mass of iron in the $3.12\text{ g}$ sample of steel.
[The relative atomic mass of iron is 56.]

mass of iron in $3.12\text{ g}$ of the steel = ______ $\text{g}$

(f)

Using your answer from (e), calculate the percentage by mass of iron in the steel.

percentage by mass of iron in the steel = ______ %

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Q2Qualitative Analysis · Experimental Contexts23M

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