Preparation Of Salts - Chemistry Worksheet Page 2
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4CHEMISTRY Class Notes
Preparation of Salts
Precipitation
1.
(the only method to prepare insoluble salts)
· Salt to prepare is an insoluble salt in solution, both reagents are aqueous solutions.
a) Starting materials for making lead(II) sulfate can be: lead(II) nitrate and sulfuric acid.
Word equation: lead(II) nitrate + sulfuric acid à lead(II) sulfate + nitric acid
(aq) à PbSO
Chemical equation: Pb(NO
)
(aq) + H
SO
(s) + 2HNO
(aq)
3
2
2
4
4
3
b) Add the reagents together and keep stirring until no more precipitation forms.
c) Filter the mixture to get the residue.
d) Wash the residue with distilled water to remove impurities.
e) Dry on a piece of filter paper to obtain the pure sample.
2. Acid + Insoluble Metal
(to prepare soluble salts)
· For moderately reactive metals like magnesium, aluminium and zinc.
· Not for unreactive metals like Cu and Ag.
· Not for very reactive metals like K, Na and Ca.
a) Starting materials for making zinc sulfate can be: zinc and sulfuric acid.
Word equation: zinc + sulfuric acid à zinc sulfate + hydrogen
(aq) à ZnSO
Chemical equation: Zn (s) + H
SO
(aq) + H
(g)
2
4
4
2
b) Add excess zinc to sulfuric acid until there is no more effervescence.
Reason: To ensure a complete reaction and all sulfuric acid has been used up, so that it
will not contaminate the desired solution.
c) Filter off the excess zinc (insoluble metal).
Reason: To ensure that the unreacted zinc will not contaminate the salt formed.
d) Heat the filtrate to saturate the zinc sulfate salt solution. Do not heat to dryness.
Reason: By heating to dryness, the salt loses its water of crystallization and crystals will
not form.
e) When filtrate is saturated, let it cool and crystallize.
f) Wash the zinc sulfate crystals with distilled water dry between pieces of filter paper.
3. Acid + Insoluble Base
(to prepare soluble salts)
a) Starting materials for making copper (II) sulfate can be: copper(II) oxide and sulfuric
acid.
Word equation: Copper(II) oxide + Sulfuric acid à Copper (II) Sulfate + Water
(aq) à CuSO
Chemical equation: CuO (s) + H
SO
(aq) + H
O (l)
2
4
4
2
b) Add excess copper(II) oxide to sulfuric acid until there is no more reaction.
Reason: To ensure a complete reaction and all sulfuric acid has been used up, so that it
will not contaminate the desired solution.
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