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Problem 1 - Entrance Test

The complex [Co(NH₃)₅Cl]SO₄ on reaction with AgNO₃ in aqueous solution gives 1 mole of AgCl per mole of the complex, while [Co(NH₃)₅Cl]Cl₂ gives 2 moles of AgCl per mole of the complex. When the isomer of [Co(NH₃)₅Cl]SO₄ (which gives 2 moles of AgCl with AgNO₃) is treated with concentrated H₂SO₄ and the resulting ester is hydrolyzed, the product is: (A) [Co(NH₃)₅SO₄]⁺ (B) [Co(NH₃)₅(H₂O)]³⁺ (C) [Co(NH₃)₅(OSO₃H)]²⁺ (D) [Co(NH₃)₅(H₂O)]³⁺ + H₂SO₄

Correct: B

The isomer of [Co(NH₃)₅Cl]SO₄ that gives 2 moles of AgCl is [Co(NH₃)₅SO₄]Cl. Upon treatment with concentrated H₂SO₄, the Cl⁻ ion is replaced by HSO₄⁻ (or SO₃), and upon hydrolysis (heating with water), the sulfate ester group is hydrolyzed to give an aqua complex. The product is [Co(NH₃)₅(H₂O)]³⁺ (the ionization isomer has its sulfate coordinated; upon hydrolysis, water replaces the coordinated group). The correct answer is [Co(NH₃)₅(H₂O)]³⁺, which corresponds to choice B.