Question

A coordination compound CrCl\(_3 \cdot 6H_2O\) is mixed with excess AgNO\(_3\) solution, two moles of AgCl are precipitated per mole of the compound. Write the structural formula of the coordination compound.

Hint:

AgNO\(_3\) test → counts free halide ions outside coordination sphere.

Answer 1

Concept: AgNO\(_3\) test is used to determine the number of ionisable chloride ions present outside the coordination sphere.
Step 1: Analyze given data.

• Total chlorides in compound = 3
• 2 moles of AgCl formed → 2 ionisable Cl\(^-\)

So:

• 2 Cl\(^-\) outside coordination sphere
• 1 Cl\(^-\) inside coordination sphere

Step 2: Assign water molecules. Total water molecules = 6 Remaining ligands inside coordination sphere: \[ 6 - 0 = 6 \text{ waters available} \] Chromium(III) typically shows coordination number = 6. Thus, inner sphere: \[ [Cr(H_2O)_5Cl]^{2+} \]
Step 3: Write full structural formula. Two chloride ions outside: \[ \boxed{[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O} \]
Explanation:

• 1 Cl inside sphere
• 2 Cl outside → gives 2 AgCl
• Remaining 1 water as lattice water