Making sodium, potassium and ammonium salts
3 marks · EasyA student wants to prepare pure potassium chloride crystals from potassium hydroxide solution and hydrochloric acid using titration.
State the first step the student should carry out before adding acid.
Describe how the student should use an indicator to find the endpoint of the reaction.
Once the volumes have been determined, explain what the student must do to obtain pure potassium chloride 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) Pipette a measured volume of potassium hydroxide solution into a conical flask.
(b) Add a few drops of indicator (e.g., methyl orange) and titrate with hydrochloric acid until the indicator changes colour at the endpoint.
(c) Repeat the reaction using the same volumes of acid and alkali, but without indicator, then evaporate the solution to obtain crystals and dry them.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States correct initial step: pipette alkali (potassium hydroxide solution) into a conical flask.
B1 (b) · 1 mark · Describes adding indicator and titrating acid until colour change/end point.
B1 (c) · 1 mark · Explains to repeat using measured volumes without indicator, crystallise, and dry the product.
Worked solution
a) Use a pipette to measure a set volume of potassium hydroxide and transfer it into a conical flask.
b) Add a few drops of methyl orange indicator, then add hydrochloric acid from a burette, swirling until the indicator changes from yellow to orange.
c) Repeat the reaction using the same measured volumes of acid and alkali, but do not add indicator. Then evaporate the resulting solution to obtain crystals, which are then dried.