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Question

How would you distinguish between Zn2+ and Pb2+ using ammonium hydroxide solution?


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Solution

Distinguishing between Zn2+and Pb2+:-

  • When Zn2+reacts with Ammonium hydroxide(NH4OH), it produces a white gelatinous ppt of Zinc hydroxideZn(OH)2.
  • The balanced reaction is:

Zn2+(s)Zincions+NH4OH(aq)AmmoniumhydroxideZn(OH)2(s)()Zinchydroxide+2NH4+(aq)Ammoniumions

  • This white ppt formed of Zinc hydroxide dissolves in excess addition of Ammonium hydroxide, that is it is soluble in excess Ammonium hydroxide
  • When Pb2+reacts with Ammonium hydroxide(NH4OH), it produces a chalky white ppt of Lead hydroxidePb(OH)2.
  • The balanced reaction is:

Pb2+(s)Leadions+NH4OH(aq)AmmoniumhydroxidePb(OH)2(s)()Leadhydroxide+2NH4+(aq)Ammoniumions

  • This chalky white ppt of Lead hydroxide does not dissolve in excess Ammonium hydroxide, that is it's insoluble in excess Ammonium hydroxide.

Therefore, Zn2+when reacts with Ammonium hydroxide, it produces a white gelatinous ppt of Zinc hydroxide which is soluble in excess Ammonium hydroxide, whereas, Pb2+ when reacts with Ammonium hydroxide, produces a chalky white ppt of Lead hydroxide which is insoluble in excess Ammonium hydroxide.


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