Problem 7 - Entrance Test

The oxidation state of chromium in the compound K₃[Cr(C₂O₄)₃]·3H₂O is +3. When this compound is dissolved in acidic solution and treated with AgNO₃, the number of moles of AgCl precipitated per mole of the complex is: (A) 0 (B) 1 (C) 2 (D) 3

A. 0B. 1C. 2D. 3

Correct: A

In K₃[Cr(C₂O₄)₃]·3H₂O, the oxalate (C₂O₄²⁻) ligands are coordinated to Cr³⁺ through the oxygen atoms. Since the complex is anionic ([Cr(C₂O₄)₃]³⁻) and the oxalate is a bidentate ligand that coordinates in a chelate manner, the oxalate ions are not ionizable (they are not counter ions). Upon dissolution in acidic solution, the coordinated oxalate does not dissociate to release Cl⁻ (there is no Cl⁻ in the compound). However, the question might be referring to the isomer where oxalate can be displaced. In the given compound, there are no chloride ions, so no AgCl is precipitated. The answer is 0 (choice A). If the question implies an isomer or a different compound, but based on the given formula, the answer is A.