On treatment of 100 mL of 0.1 M solution of CoCl3.6H2O with excess AgNO3, 1.2 × 10²² ions are precipitated. The complex is:

Avogadro's number is 6.023 × 10²³

Number of moles of CoCl3.6H2O = 0.1 M × 0.1 L = 0.01 moles

Number of Cl⁻ ions precipitated = 1.2 × 10²² ions

Number of moles of Cl⁻ ions precipitated = (1.2 × 10²²) / (6.023 × 10²³) = 0.0199 moles ≈ 0.02 moles

Since 0.01 moles of CoCl3.6H2O gives 0.02 moles of precipitated Cl⁻ ions, it means that each mole of CoCl3.6H2O gives 2 moles of free Cl⁻ ions.

Therefore, the complex ion must have 1 Cl⁻ ion coordinated to the Co³⁺ ion and 2 Cl⁻ ions outside the coordination sphere.

The correct formula is [Co(H2O)5Cl]Cl2.H2O