Question

The molar conductivity of the complex CoCl$_3$ · 4NH$_3$ · 2H$_2$O is found to be the same as that of a 1 : 3 electrolyte. The structural formula of the compound is :

• A. [Co(NH$_3$)$_4$(H$_2$O)$_2$]Cl$_3$
• B. [Co(NH$_3$)$_4$Cl (H$_2$O)]Cl$_2$H$_2$O
• C. [Co(NH$_3$)$_4$ (H$_2$O)Cl$_2$]ClH$_2$O
• D. [Co(NH$_3$)$_4$(H$_2$O)$_2$Cl]Cl$_2$

Answer 1

Answer: (A)

[Co(NH$_3$)$_4$(H$_2$O)$_2$]Cl$_3$

Answer 2

The molar conductivity of a complex is related to the number of ions it produces in solution. A 1:3 electrolyte dissociates into a total of $1+3=4$ ions per formula unit. The given complex is $\text{CoCl}_3 \cdot 4\text{NH}_3 \cdot 2\text{H}_2\text{O}$. We need to find the structural formula where the complex dissociates into 4 ions.

• Option A: $[\text{Co(NH}_3)_4(\text{H}_2\text{O})_2]\text{Cl}_3$: This complex dissociates into one complex cation $[\text{Co(NH}_3)_4(\text{H}_2\text{O})_2]^{3+}$ and three chloride anions $\text{Cl}^-$. The total number of ions is $1 + 3 = 4$. This corresponds to a 1:3 electrolyte.

• Option B: $[\text{Co(NH}_3)_4\text{Cl}(\text{H}_2\text{O})]\text{Cl}_2\text{H}_2\text{O}$: This complex dissociates into one complex cation $[\text{Co(NH}_3)_4\text{Cl}(\text{H}_2\text{O})]^{2+}$ and two chloride anions $\text{Cl}^-$. The total number of ions is $1 + 2 = 3$. This corresponds to a 1:2 electrolyte.

• Option C: $[\text{Co(NH}_3)_4 (\text{H}_2\text{O})\text{Cl}_2]\text{ClH}_2\text{O}$: This complex dissociates into one complex cation $[\text{Co(NH}_3)_4 (\text{H}_2\text{O})\text{Cl}_2]^{+}$ and one chloride anion $\text{Cl}^-$. The total number of ions is $1 + 1 = 2$. This corresponds to a 1:1 electrolyte.

• Option D: $[\text{Co(NH}_3)_4(\text{H}_2\text{O})_2\text{Cl}]\text{Cl}_2$: This complex dissociates into one complex cation $[\text{Co(NH}_3)_4(\text{H}_2\text{O})_2\text{Cl}]^{2+}$ and two chloride anions $\text{Cl}^-$. The total number of ions is $1 + 2 = 3$. This corresponds to a 1:2 electrolyte.

Comparing the number of ions produced, only option Ⓐ produces 4 ions, which is consistent with a 1:3 electrolyte.