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

Step 1: Understanding the Compound.
The given compound is CrCl₃·6H₂O. This indicates that the compound consists of chromium chloride (CrCl₃) coordinated with six water molecules (6H₂O) as ligands.

Step 2: Reaction with AgNO₃.
When the compound CrCl₃·6H₂O reacts with excess AgNO₃ solution, two moles of AgCl are precipitated per mole of the compound. This suggests that two of the chloride ions are **free** and are not coordinated to the chromium ion. The remaining chloride ions must be coordinated to the chromium ion.

Step 3: Determine the Coordination Complex.
The chromium ion (Cr) has a charge of +3 (since CrCl₃ is neutral, the chromium ion must have a +3 charge to balance the 3 Cl⁻ ions). Since two chloride ions are free, the remaining chloride ions are coordinated with the chromium ion. Additionally, six water molecules are coordinated as ligands.

Step 4: Structural Formula of the Coordination Compound.
The structural formula of the coordination compound can be written as: \[ [CrCl_2(H_2O)_4]Cl \cdot 6H_2O \] Here: - \( [CrCl_2(H_2O)_4] \) represents the coordination sphere where two chloride ions and four water molecules are coordinated to the chromium ion. - The \(Cl\) outside the coordination sphere is the free chloride ion, which is precipitated as AgCl. - The \( \cdot 6H_2O \) indicates that six water molecules are present as hydration molecules, not coordinated to the chromium but associated with the crystal structure.

Conclusion.
The structural formula of the coordination compound is \( [CrCl_2(H_2O)_4]Cl \cdot 6H_2O \).