Transcript Slide 1

Identifying ions present
in solution
A colourless solution is analysed to determine the cation and
anion present.
To separate samples of this solution various tests were carried
out and the observations were recorded as follows.
•Turns red litmus blue
•Does not bubble with the addition of dilute HCl
•no reaction with aqueous barium nitrate
•a white precipitate formed when a small volume of
aqueous sodium hydroxide was added and this remained
when excess sodium hydroxide was added
•a colourless solution formed with dilute sulfuric acid
Using these observations identify the cation and anion present
in the solution.
Write balanced equations for the precipitation reactions that
occurred and for the formation of any complex ions. Link these
equations to the observations recorded.
The unknown anion is OH
• Identified with litmus (changes red blue)
• No reaction with HCL
The unknown cation is Mg2+
• White ppt with NaOH that stays with
excess
• Mg2+ + OH-  Mg(OH)2(s)
• Adding H2SO4 colourless so Mg2+
Anion
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No colour change with red litmus - not CO32 or OH
No reaction when AgNO3 added - not Cl
No reaction when BaCl2 added - not SO42
therefore anion must be NO3, (no equations since no reactions)
Cation
• A white precipitate formed when a small volume of aqueous sodium
hydroxide was added and dissolved when excess sodium hydroxide
was added  must be Pb2+ or Zn2+ or Al3+
• A white precipitate formed when a small volume of aqueous silver
nitrate was added and dissolved when excess silver nitrate was
added  must be Pb2+ or Al3+
• a white precipitate formed with dilute sulfuric acid  cation must be
Pb2+
• Pb2+(aq) + 2 OH(aq)  Pb(OH)2(s) lead hydroxide
• Pb(OH)2(s) + 2 OH(aq)  [Pb(OH)4]2(aq)
• or Pb2+(aq) + 4 OH(aq)  [Pb(OH)4]2(aq)
• SO42(aq) + Pb2+(aq)  PbSO4(s) lead sulfate
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A colourless solution is analysed to determine the cation
and anion present.
To separate samples of this solution various tests were
carried out and the observations were recorded as
follows.
Turns red litmus blue
no reaction with Hydrochloric acid
a orange precipitate formed when a small volume of
aqueous sodium hydroxide was added.
When a few drops of KSCN solution were added to a
new sample a dark red solution formed
Using these observations identify the cation and anion
present in the solution.
Write balanced equations for the precipitation reactions
that occurred and for the formation of any complex ions.
Link these equations to the observations recorded.
The unknown solution is Fe(OH)3
The unknown cation is Fe3+
• addition of NaOH produces an orange precipitate
• Fe3+(aq) + 3 OH(aq)  Fe(OH)3(s)
• Addition of Thiocyanate produces a Blood red
solution.
• Fe3+(aq) + SCN(aq)  [FeSCN] (aq)
The unknown anion is OH
• Identified with litmus (changes red blue)
• No reaction with HCL
A colourless solution is analysed to determine the cation and anion
present.
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To separate samples of this solution various tests were carried out
and the observations were recorded as follows:
no effect on red litmus
no reaction with aqueous silver nitrate
no reaction with aqueous barium nitrate
a white precipitate formed when a small volume of aqueous
sodium hydroxide was added and this disappeared when
excess sodium hydroxide was added
a white precipitate formed with dilute sulfuric acid
a white precipitate formed when a small volume of aqueous
ammonia was added and this remained in the presence of
excess ammonia.
Using these observations identify the cation and anion present in the
solution.
Write balanced equations for the precipitation reactions that
occurred and for the formation of any complex ions. Link these
equations to the observations recorded.