It was found that some of the chloride ions could be precipitated as AgCl by adding excess silver nitrate solution in compound [Co(NH3)6]3+3Cl−. The solution conductivity corresponds to
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1:3
According to Werner’s theory, a metal in a coordination compound has two kinds of valency. The secondary valency is satisfied by ligands bound directly to the metal inside the coordination sphere; these do not ionise. The primary valency is satisfied by ions outside the sphere; these ionise in water. Only the chloride ions outside the sphere are free to react with silver nitrate:
\( \displaystyle \ce{Ag^+ + Cl^- -> AgCl v} \)
In the given compound all six \( \ce{NH3} \) are in the coordination sphere, so each formula unit has 3 ionisable Cl− and gives 3 mol of AgCl per mole of \( \ce{[Co(NH3)6]Cl3} \):
\( \displaystyle \ce{[Co(NH3)6]Cl3 -> [Co(NH3)6]^3+ + 3Cl^-} \)
It gives one cation and three anions, so its conductivity corresponds to a 1 : 3 electrolyte.
Why the other options are incorrect
Option (1) 1:2 — Incorrect. This would be \( \ce{CoCl3} \).\( \ce{5NH3} \), i.e. \( \ce{[Co(NH3)5Cl]Cl2} \), which gives 2 mol AgCl per mol.
Option (3) 1:4 — Incorrect. The compound has only three counter ions, not four.
Option (4) 1:1 — Incorrect. This would be \( \ce{CoCl3} \).\( \ce{4NH3} \), i.e. \( \ce{[Co(NH3)4Cl2]Cl} \), which gives only 1 mol AgCl per mol.
NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 07-06 S2 19-05 S2 | – | – | – | – |