Making sodium, potassium and ammonium salts
2 marks · HardGive two steps the student must follow to make pure potassium chloride crystals using titration and crystallisation with potassium hydroxide and hydrochloric acid.
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Must show: Carry out a titration using indicator to find the correct volumes of acid and alkali needed for neutralisation., Repeat the neutralisation using the correct volumes of acid and alkali without indicator, then evaporate the solution to crystallise potassium chloride.
Also accept: 1. Use methyl orange to titrate potassium hydroxide with hydrochloric acid, recording the volumes needed for neutralisation. 2. Mix the same volumes of potassium hydroxide and hydrochloric acid without indicator, then evaporate water to obtain crystals.; 1. Carry out a titration with indicator to find the right amounts of potassium hydroxide and acid, 2. Repeat using those volumes (no indicator), concentrate solution and crystallise salt.
Common mistakes: Describing the 'add excess solid, then filter' method.; Leaving out the need to repeat without indicator.; Not mentioning crystallisation.
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Marking points
M1 · 1 mark · Describes performing titration using indicator to determine correct volumes for neutralisation.
A1 · 1 mark · States that the correct volumes are mixed again without indicator, followed by crystallisation (evaporation or slow crystallisation) to obtain pure potassium chloride.
Worked solution
1. Perform a titration by adding hydrochloric acid to potassium hydroxide with methyl orange to find the exact volume needed to reach the endpoint.
2. Mix those same volumes of potassium hydroxide and hydrochloric acid again without indicator, then evaporate water from the solution so crystals of potassium chloride form.