The secondary valency of the compound NiCl2.6H2O if it precipitates 2 moles of AgCl with excess of AgNO3 is.........
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Six
By Werner’s theory, the secondary valency of a metal is non-ionisable and equals its coordination number; only ions outside the coordination sphere are ionisable.
Only the chloride ions outside the coordination sphere are free in solution and precipitate as AgCl. \( \ce{NiCl2} \).\( \ce{6H2O} \) gives 2 moles of AgCl, so both chloride ions are ionisable. The six water molecules must then be the ligands, and the compound is \( \ce{[Ni(H2O)6]Cl2} \):
\( \displaystyle \ce{[Ni(H2O)6]Cl2 + 2AgNO3 -> [Ni(H2O)6](NO3)2 + 2AgCl v} \)
So the primary valency of nickel is 2 (\( \ce{Ni^{2+}} \)), and its secondary valency is six.
Why the other options are incorrect
Option (1) Two — Incorrect. This is the primary valency (oxidation state) of nickel, satisfied by the two ionisable \( \ce{Cl^{-}} \) ions.
Option (3) Eight — Incorrect. This adds the six ligands and the two chloride ions together. The chloride ions are outside the coordination sphere and do not count.
Option (4) Four — Incorrect. A coordination number of four would need only four ligands on nickel, but all six water molecules are bonded to it here.
NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – …
