Making sodium, potassium and ammonium salts
4 marks · NormalA student needs to prepare pure sodium sulfate crystals using a titration of sulfuric acid and sodium hydroxide solutions.
State the first step the student should take before starting the titration.
Describe briefly how the student would use titration to find the correct volume of acid to neutralise the alkali.
After identifying the correct volumes, outline two steps to obtain pure sodium 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) Transfer a measured volume of sodium hydroxide solution to a conical flask. · must show measured amount of alkali placed in flask
(b) Add methyl orange indicator, then add dilute sulfuric acid from the burette until the indicator just changes colour, noting the volume of acid used. · must show indicator added, acid added from burette until colour change, endpoint
(c) Mix the same volumes of sodium hydroxide and sulfuric acid in a clean flask without indicator; evaporate water (to crystallisation), cool to form crystals; filter and dry crystals. · must show mix solutions without indicator, crystallise (evaporate, cool, filter, dry)
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that student should measure (pipette) a known volume of sodium hydroxide into flask.
B2 (b) · 1 mark · Mentions adding methyl orange, titrating with acid until end point (colour change/orange) and recording volume.
B3a (c) · 1 mark · Describes mixing the same volumes of acid and alkali again without indicator.
B3b (c) · 1 mark · Describes evaporating the solution to crystallisation, cooling, filtering and drying.
Worked solution
(a) Measure a set volume (e.g., 25 cm³) of sodium hydroxide solution into a conical flask.
(b) Add a few drops of methyl orange indicator. Titrate with sulfuric acid from a burette, swirling, until the indicator changes from yellow to orange. Note the volume of acid added.
(c) Mix these volumes of acid and alkali in a clean flask without indicator. Heat the solution to evaporate some water to make it saturated, cool to allow crystals to form, then filter, and dry the sodium sulfate crystals.