The table shows Werner’s formulation of four cobalt(III) chloride–ammonia complexes.
Aqueous solutions of the four complexes are prepared at the same molar concentration and the same temperature. Which of the following shows the correct order of their molar conductivity?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Yellow > Purple > Green = Violet
In the same solvent and at the same temperature, the conductivity of electrolyte solutions depends on the charge of the ions and the concentration of ions. In each complex the atoms inside the square brackets stay together as one ion; only the chloride ions outside the brackets are set free as separate ions. So, although every complex has three chlorine atoms per cobalt, the number of ions is not the same.
Yellow — \( \ce{[Co(NH3)6]^3+ 3Cl-} \), a 1:3 electrolyte. One formula unit gives 4 ions, and the complex cation carries a 3+ charge, so its molar conductivity is the highest.
Purple — \( \ce{[CoCl(NH3)5]^2+ 2Cl-} \), a 1:2 electrolyte. One formula unit gives 3 ions, and the complex cation carries a 2+ charge.
Green and Violet — both \( \ce{[CoCl2(NH3)4]^+ Cl-} \), 1:1 electrolytes. Each formula unit gives 2 ions of the same kinds, so their molar conductivities are equal and the lowest of the four.
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “These observations, together with the results of conductivity measurements in solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 5.1, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions.”
2.4.1 Measurement of the Conductivity of Ionic Solutions – “The conductivity of solutions of different electrolytes in the same solvent and at a given temperature differs due to charge and size of the ions in which they dissociate, the concentration of ions or ease with which the ions move under a potential gradient.”
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes