Making sodium, potassium and ammonium salts
5 marks · HardA student is asked to prepare pure ammonium sulfate crystals using ammonia solution and dilute sulfuric acid in the laboratory.
Explain why the titration method must be used instead of the excess solid method for this preparation.
State two practical reasons why an indicator is not added to the acid and alkali mixture when making the final batch of ammonium sulfate for crystallisation.
Describe what the student should do after mixing the correct volumes of acid and alkali, to obtain pure, dry crystals of ammonium sulfate.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show All ammonium compounds are soluble, so excess solid cannot be removed by filtration; no visible excess forms., There is no way to know when all acid has reacted, as extra solid dissolves completely.
(b) — · must show Indicator would contaminate the salt/crystals., Indicator is not needed—correct volumes are known from titration., To keep the product pure (no coloured impurities)., Indicators are added only to the trial titration.
(c) — · must show Heat the solution to evaporate water and form crystals, then filter and dry the crystals., Evaporate water, crystallise, filter, dry.
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Marking points
M1 (a) · 1 mark · Correctly states that all ammonium compounds are soluble, so no excess solid forms or can be removed by filtration.
A1 (a) · 1 mark · Explains that it is not possible to tell when all acid has reacted, as any extra reagent will simply dissolve.
B1 (b) · 1 mark · States that indicator is not added to avoid contamination (product purity).
B2 (b) · 1 mark · States that indicator is unnecessary because the correct volumes are already known from the titration.
C1 (c) · 1 mark · Describes process: evaporate water, allow to crystallise, then filter and dry crystals.
Worked solution
a) The titration method is used because all ammonium compounds are soluble in water, so any excess reagent just dissolves. There is no visible solid to filter off, so you cannot tell when neutralisation is complete without titration.
b) The indicator is not added to keep the product pure (avoid coloured impurities) and because the correct volumes of acid and alkali have already been determined in the titration, so it is unnecessary.
c) After mixing, the solution should be heated gently to evaporate some water until crystals form, then the crystals are filtered and dried.