Acids, Bases and Salt Preparations
5 marks · EasyThis question is about the formation of crystals when preparing soluble salts.
Explain why crystallisation is used instead of evaporating to dryness when making hydrated copper(II) sulfate crystals.
Describe, in order, the steps taken after completing a titration to obtain pure sodium sulfate crystals from sodium hydroxide and sulfuric acid.
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) Crystallisation allows water of crystallisation to be included; evaporating to dryness gives the anhydrous salt, not hydrated crystals.
(b) After titration, mix the correct volumes of acid and alkali (without indicator), heat the solution to crystallisation point, allow to cool for crystals to form, then filter and dry the crystals.
Common mistakes: Not mentioning water of crystallisation; Omitting reheating or cooling steps; Reversing or missing steps
Show mark scheme and worked solution
Marking points
E1 (a) · 1 mark · States that crystallisation keeps water of crystallisation or produces hydrated crystals.
E2 (a) · 1 mark · States that evaporating to dryness produces anhydrous salt (not hydrated crystals) or loses water of crystallisation.
E3 (b) · 1 mark · States to repeat the titration with correct volumes of acid and alkali, but without indicator.
E4 (b) · 1 mark · Mentions concentrating the solution by heating (until near saturation) and allowing to cool for crystals to form.
E5 (b) · 1 mark · Mentions separating the crystals by filtration and drying them.
Worked solution
a) Crystallisation is used because it allows water from the solution to become part of the salt structure, forming hydrated copper(II) sulfate. Evaporating to dryness gives anhydrous copper(II) sulfate (white), which does not contain water of crystallisation.
b) After the titration shows the correct volume of acid needed, repeat the mixing of acid and alkali in clean apparatus but without the indicator. Heat the resulting solution to concentrate it (but do not boil dry), then allow it to cool so that sodium sulfate crystals can form. Finally, filter off the crystals and dry them using a paper towel or in a warm place.