The hydrate isomers of CrCl3·6H2O cannot be distinguished by:

The correct answer is Option (4) → Magnetic moment

Hydrate isomers differ in whether a water molecule is bonded to the metal ion or is only present in the crystal lattice. For \( \ce{CrCl3.6H2O} \), \( \ce{[Cr(H2O)6]Cl3} \) (violet) and \( \ce{[Cr(H2O)5Cl]Cl2.H2O} \) (grey-green) are two such isomers. A method separates them only if it senses the change in the ligands or in the ions outside the coordination sphere.

Option (1) Conductivity measurements — Can distinguish them. \( \ce{[Cr(H2O)6]Cl3} \) gives 4 ions in solution, \( \ce{[Cr(H2O)5Cl]Cl2} \) only 3, so their molar conductivities differ.

Option (2) Precipitation by AgNO₃ — Can distinguish them. Only the chloride ions outside the coordination sphere are precipitated: 3 mol AgCl per mole from the violet isomer, 2 mol from the grey-green one.

Option (3) Colour — Can distinguish them. The isomers are violet and grey-green, since the ligands around \( \ce{Cr^3+} \) are different.

Option (4) Magnetic moment — Cannot distinguish them. Every isomer has chromium in the +3 state:

• Cr (Z = 24) is [Ar]3d⁵4s¹; \( \ce{Cr^3+} \) is 3d³.

• In an octahedral complex the three electrons occupy the three t_2g orbitals singly, whatever the ligands: n = 3.

• μ = \( \sqrt{3\times5} \) = 3.87 BM for every isomer.

NCERT (2026-27 edition):
5.4.6 Solvate Isomerism – “This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. … Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂.H₂O (grey-green).”