Making sodium, potassium and ammonium salts
3 marks · EasyA student prepares pure potassium sulfate crystals from potassium hydroxide and sulfuric acid using titration.
State the purpose of using an indicator during the titration.
After finding the correct volumes, outline two steps the student must take to obtain pure potassium sulfate crystals.
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.
Show answer
(a) To show when neutralisation is complete / to determine the endpoint of the titration.
(b) Mix the correct volumes of acid and alkali again without indicator; evaporate the solution to form crystals (allow 'concentrate, then leave to cool and crystallise').
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States the purpose of the indicator: to show endpoint or completion of reaction.
M1 (b) · 1 mark · States the need to repeat mixing correct volumes without indicator.
M2 (b) · 1 mark · States any of: evaporate/concentrate the solution to form crystals, leave to crystallise, cool to form crystals (accept alternative correct wording).
Worked solution
a) The indicator shows when all the alkali has reacted – that is, when neutralisation is complete.
b) The student should first mix the exact volumes of potassium hydroxide and sulfuric acid again without any indicator. Then, they evaporate the water from the solution until crystals begin to form, or until a saturated solution is obtained and leave it to cool so crystals appear.