Statement I: [Co(en)2(NH3)Br]SO4 has a lower electrical conductivity in solution than [Co(en)2(NH3)(SO4)]Br.
Statement II: In solution, [Co(en)2(NH3)Br]SO4 and [Co(en)2(NH3)(SO4)]Br give the same number of ions per formula unit.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Statement I is incorrect but Statement II is correct.
The two compounds are ionisation isomers: the \( \ce{Br^-} \) and \( \ce{SO4^2-} \) groups swap places between the coordination sphere and the counter ion. In solution, only the counter ion separates from the complex entity:
\( \displaystyle \ce{[Co(en)2(NH3)Br]SO4 -> [Co(en)2(NH3)Br]^2+ + SO4^2-} \)
\( \displaystyle \ce{[Co(en)2(NH3)(SO4)]Br -> [Co(en)2(NH3)(SO4)]^+ + Br^-} \)
Statement I: [Co(en)2(NH3)Br]SO4 has a lower electrical conductivity in solution than [Co(en)2(NH3)(SO4)]Br. — False. Both give two ions, but the first gives doubly charged ions (2+ and 2−) while the second gives singly charged ones. A doubly charged ion carries more charge and conducts more: the sulphate ion has a limiting molar conductivity of 160.0 S cm2 mol−1 against 78.1 for the bromide ion. So the first compound conducts more, not less.
Statement II: In solution, [Co(en)2(NH3)Br]SO4 and [Co(en)2(NH3)(SO4)]Br give the same number of ions per formula unit. — True. Each gives one complex ion and one counter ion, as the equations show.
NCERT (2026-27 edition):
5.4.5 Ionisation Isomerism – “This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.”
2.4.2 Variation of Conductivity and Molar Conductivity with Concentration – “… The law states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte. … The values of λ° for some cations and anions at 298 K are given in Table 2.4.”
Table 2.4: Limiting Molar Conductivity for some Ions in Water at 298 K
