Question 2

FeSO4 solution mixed with (NH4)2SO4 solution in 1:1 molar ratio gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous ammonia in 1:4 molar ratio does not give the test of Cu2+ ion. Explain why?

Answer

Both the compounds i.e., FeSO4 .(NH4)2 SO4 .6H2O and [Cu(NH3)4] SO4 .5H2O fall under the category of addition compounds with only one major difference i.e., the former is an example of a double salt, while the latter is a coordination compound.



A double salt is an addition compound that is stable in the solid state but that which breaks up into its constituent ions in the dissolved state. These compounds exhibit individual properties of their constituents. For e.g. FeSO4 .(NH4)2 SO4 .6H2O breaks into Fe2+, NH4+, and SO42 - ions. Hence, it gives a positive test for Fe2+ ions.

A coordination compound is an addition compound which retains its identity in the solid as well as in the dissolved state. However, the individual properties of the constituents are lost. This happens because [Cu(NH3)4] SO4 .5H2O does not show the test for Cu2+. The ions present in the solution of [Cu(NH3)4] SO4 .5H2O are [Cu(NH3)4]2+  and SO42-.

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