Acids, Bases and Salt Preparations
5 marks · EasyThis question is about preparing soluble salts using different methods.
State why it is necessary to use an excess of the solid when preparing copper(II) sulfate from copper(II) oxide and sulfuric acid.
Give one reason why you do not use the same 'excess solid' method for making sodium sulfate using sodium hydroxide and sulfuric acid.
Give the name of the technique used instead to make sodium sulfate from sodium hydroxide and sulfuric acid.
Explain briefly how you know when to stop adding acid during the preparation of sodium sulfate by this technique.
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.
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(a) To ensure all the acid reacts/neutralises so no acid remains in the filtrate.
(b) Because all sodium salts are soluble; excess solid would dissolve making it impossible to separate by filtration.
(c) Titration
(d) By using an indicator such as methyl orange to show the endpoint; the indicator changes colour when neutralisation is complete.
Common mistakes: Confusing reasons for excess/filtration; Incorrectly naming the titration method; Vague references to indicator
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States the need for excess to ensure all acid is neutralised (no acid left).
B2 (b) · 1 mark · Correctly states that all sodium salts are soluble so excess solid cannot be separated by filtration.
B3 (c) · 1 mark · Names the method as 'titration'.
B4 (d) · 1 mark · States indicator is used to find when neutralisation is complete (using colour change).
B5 (d) · 1 mark · Gives methyl orange (or correct indicator) and explains what the colour change means (endpoint reached).
Worked solution
a) An excess of solid is used so that all the acid has completely reacted. Any acid left would make the crystals impure.
b) For sodium salts, all sodium compounds are soluble so the excess solid dissolves. There is no visible excess to filter off, so this method cannot be used.
c) The technique used instead is titration.
d) The acid is added to alkali with an indicator (such as methyl orange) present. The endpoint is reached when the indicator changes colour (from yellow to orange for methyl orange), showing that the acid and alkali have completely reacted.