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Question

You are provided with PbCO3, ZnCO3 and CaCO3. How will you identify these cations?

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Solution

The cations in PbCO3, ZnCO3 and CaCO3 are determined with the help of reaction with ammonium hydroxide.
Reaction with ammonium hydroxide
Lead carbonate Zinc carbonate Calcium carbonate
Dissolve a small amount of lead carbonate in dilute nitric acid. Add NH4OH to this solution. A white precipitate of Pb(OH)2 is formed, which is insoluble in excess of ammonium hydroxide.
PbCO3 + 2NH4OH → Pb(OH)2 + (NH4)2CO3

Dissolve a small amount of zinc carbonate in dilute nitric acid. Add NH4OH to this solution. A white gelatinous precipitate of Zn(OH)2 is formed, which is soluble in excess of ammonium hydroxide.
ZnCO3 + 2NH4OH → Zn(OH)2 + (NH4)2CO3

Zn(OH)2 + (NH4)2CO3 + 2NH4OH → [Zn(NH3)4]CO3 + 4H2O
Dissolve a small amount of calcium carbonate in dilute nitric acid. Add NH4OH to this solution. No reaction takes place.

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