Which among the following are coordination complexes?
(A) Mohr's salt
(B) Tetraamminediaquacobalt (II) ion
(C) Carnallite
(D) Potassium ferricyanide
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
Both double salts and coordination compounds are formed when two or more stable compounds combine in a fixed ratio. The difference shows up in water: a double salt dissociates completely into simple ions, while a complex ion keeps its identity and does not break up into the metal ion and its ligands.
(A) Mohr’s salt — Incorrect. \( \ce{FeSO4.(NH4)2SO4.6H2O} \) is a double salt; in water it gives \( \ce{Fe^2+} \), \( \ce{NH4+} \) and \( \ce{SO4^2-} \) ions, all of which show their usual tests.
(B) Tetraamminediaquacobalt (II) ion — Correct. \( \ce{[Co(NH3)4(H2O)2]^2+} \) is a complex ion: four \( \ce{NH3} \) and two \( \ce{H2O} \) ligands are bonded to the central \( \ce{Co^2+} \) ion.
(C) Carnallite — Incorrect. \( \ce{KCl.MgCl2.6H2O} \) is a double salt; it dissociates into \( \ce{K+} \), \( \ce{Mg^2+} \) and \( \ce{Cl-} \) ions.
(D) Potassium ferricyanide — Correct. \( \ce{K3[Fe(CN)6]} \) gives \( \ce{K+} \) ions and the complex ion \( \ce{[Fe(CN)6]^3-} \), which does not dissociate into \( \ce{Fe^3+} \) and \( \ce{CN-} \).
NCERT (2026-27 edition):
Difference between a Double Salt and a Complex
