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Q5.2:
((FeSO_{4})) solution mixed with ((NH_{4})_{2}SO_{4}) solution in 1:1 molar ratio gives the test of ((Fe^{2+})) ion but ((CuSO_{4})) solution mixed with aqueous ammonia in 1:4 molar ratio does not give the test of ((Cu^{2+})) ion. Explain why?

Solution :

Step 1: Analyzing the Formation of Mohr's Salt

When an aqueous solution of Ferrous Sulfate ($FeSO_4$) is mixed with Ammonium Sulfate ($(NH_4)_2SO_4$) in a $1:1$ molar ratio, the resulting compound is Ferrous Ammonium Sulfate, commonly known as Mohr's Salt. The reaction is represented as follows:

$FeSO_4(aq) + (NH_4)_2SO_4(aq) + 6H_2O(l) \rightarrow FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O(s)$

Justification: [Per the definition of Double Salts], Mohr's Salt dissociates completely into its constituent simple ions when dissolved in water:

$FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O \rightleftharpoons Fe^{2+}(aq) + 2NH_4^+(aq) + 2SO_4^{2-}(aq) + 6H_2O$

Since the $Fe^{2+}$ ions are released into the solution phase, they remain available to react with chemical reagents, thereby providing a positive test for the $Fe^{2+}$ ion.

Step 2: Analyzing the Formation of Tetraamminecopper(II) Sulfate

When an aqueous solution of Copper(II) Sulfate ($CuSO_4$) is mixed with aqueous ammonia ($NH_3$) in a $1:4$ molar ratio, a coordination complex is formed. The reaction is defined by the following equilibrium:

$CuSO_4(aq) + 4NH_3(aq) \rightarrow [Cu(NH_3)_4]SO_4(aq)$

Justification: [Per Werner's Coordination Theory], the resulting species is a coordination complex. Unlike double salts, coordination complexes do not dissociate into individual constituent ions in an aqueous medium. Instead, they ionize into the complex ion and the counter ion:

$[Cu(NH_3)_4]SO_4 \rightleftharpoons [Cu(NH_3)_4]^{2+}(aq) + SO_4^{2-}(aq)$

Step 3: Comparative Analysis of Ion Availability

The stability of the coordination entity $[Cu(NH_3)_4]^{2+}$ is characterized by a very high formation constant ($K_f$). The concentration of free $Cu^{2+}$ ions in the solution is extremely low, effectively negligible according to the dissociation constant of the complex ($K_d = 1/K_f$).

Compound Classification Dissociation Behavior Test Result
Mohr's Salt Double Salt Dissociates into simple ions ($Fe^{2+}$) Positive for $Fe^{2+}$
$[Cu(NH_3)_4]SO_4$ Coordination Complex Retains complex ion $[Cu(NH_3)_4]^{2+}$ Negative for $Cu^{2+}$

Visual Representation of Coordination Geometry

Cu NH₃ NH₃ NH₃ NH₃ Square Planar structure of [Cu(NH₃)₄]²⁺

Final Solution: Mohr's salt is a double salt that dissociates completely into $Fe^{2+}$ ions in water, allowing them to be detected. Conversely, $[Cu(NH_3)_4]SO_4$ is a coordination compound that retains the $[Cu(NH_3)_4]^{2+}$ complex ion in solution, keeping the $Cu^{2+}$ concentration below the threshold of analytical detection tests.


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