Which of the following is an example of a double salt?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Mohr’s salt
A double salt is formed when two simple salts combine in a fixed ratio, but in water it dissociates completely into simple ions. A complex keeps its complex ion intact in water.
Mohr’s salt, \( \ce{FeSO4.(NH4)2SO4.6H2O} \), gives only simple ions in water:
\( \displaystyle \ce{FeSO4.(NH4)2SO4.6H2O -> Fe^2+ + 2NH4^+ + 2SO4^2- + 6H2O} \)
So it is a double salt.
Why the other options are incorrect
Option (1) Tetraamminecopper(II) sulphate — Incorrect. \( \ce{[Cu(NH3)4]SO4} \) is a coordination compound; in water the complex ion \( \ce{[Cu(NH3)4]^2+} \) stays intact and does not break into \( \ce{Cu^2+} \) and \( \ce{NH3} \).
Option (3) Potassium hexacyanidoferrate(III) — Incorrect. \( \ce{K3[Fe(CN)6]} \) gives \( \ce{K^+} \) and the complex ion \( \ce{[Fe(CN)6]^3-} \), which does not dissociate into \( \ce{Fe^3+} \) and \( \ce{CN^-} \) ions.
Option (4) Hexaamminecobalt(III) chloride — Incorrect. \( \ce{[Co(NH3)6]Cl3} \) gives \( \ce{Cl^-} \) and the complex ion \( \ce{[Co(NH3)6]^3+} \); the six \( \ce{NH3} \) ligands stay bound to cobalt.
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, KCl.MgCl2.6H2O, Mohr’s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc. dissociate into simple ions completely when dissolved in water.”