Preparation Of Inorganic Compounds (double Salts)

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Mohr's salt, prepared in the laboratory from ferrous sulphate and ammonium sulphate, behaves differently from a coordination complex when dissolved in water. What is the key reason for this behaviour?

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Mohr's salt is a double salt with the formula FeSO_4·(NH_4)_2SO_4·6H_2O, prepared by crystallising an equimolar mixture of ferrous sulphate and ammonium sulphate. Double salts are stable only in the solid crystalline state; when dissolved in water they dissociate completely into all their constituent simple ions, here Fe^{2+}, NH_4^+, and SO_4^{2-}. Consequently the solution gives the independent confirmatory tests of each ion, such as a positive test for ferrous and for ammonium ions separately. This contrasts with a coordination complex, in which a complex ion remains intact in solution and the bound metal does not give its usual free-ion tests. Option B describes a coordination complex, not a double salt, and is therefore wrong. Option C is incorrect because Mohr's salt is readily water-soluble. Option D is wrong because ammonium ions are stable in cold neutral solution and ammonia is liberated only on warming with alkali. This distinction is the standard NCERT contrast between double salts and complexes. Plausibility check: since each ion gives its own test, the bonding in solution must be ionic rather than the strong directional bonding of a complex ion.

Preparation Of Inorganic Compounds (double Salts)

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