Chemistry 5070/31 — May/June 2019

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

questions

marks

minutes

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

Q117MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationPlanning Experiments and InvestigationsFree sample

The concentration of aqueous sodium sulfite can be determined by titrating acidified 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 aqueous sodium sulfite.
Q is $0.0192\text{ mol / dm}^3$ 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 $25.0\text{ cm}^3$ portion of P into a flask. To the solution in the flask also add about $25\text{ cm}^3$ of dilute sulfuric acid using a measuring cylinder.

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

12M

(b)

Q is $0.0192\text{ mol / dm}^3$ 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)

In the titration reaction, five moles of sodium sulfite react with two moles of potassium manganate(VII).

Use your answer from (b) to calculate the number of moles of sodium sulfite present in $25.0\text{ cm}^3$ of P.

number of moles of sodium sulfite in $25.0\text{ cm}^3$ of P = ______

(d)

Use your answer from (c) to calculate the number of moles of sodium sulfite in $1.00\text{ dm}^3$ of P.

number of moles of sodium sulfite in $1.00\text{ dm}^3$ of P = ______

(e)

Use your answer from (d) to calculate the mass of sodium sulfite in $1.00\text{ dm}^3$ of P.
[ $M_r$ : $\text{Na}_2\text{SO}_3$ , 126]

mass of sodium sulfite in $1.00\text{ dm}^3$ of P = ______ $\text{g}$

(f)

Explain why the volume of dilute sulfuric acid added to P in each titration does not need to be measured accurately.

______

The rest of this paper

1 more questions

Q2Qualitative Analysis · Experimental Contexts23M

Loading the full paper…