Question

A 1:3 electrolyte in an aqueous solution is

• A. \([ \text{Co}(\text{NH}_3)_3(\text{NO}_2)_3 ]\)
• B. \([ \text{CoCl}_2(\text{NH}_3)_4 ]\text{Cl}\)
• C. \([ \text{CoCl}(\text{NH}_3)_5 ]\text{Cl}_2\)
• D. \([ \text{Co}(\text{NH}_3)_6 ]\text{Cl}_3\)

Hint:

To easily identify the type of electrolyte for a coordination complex, look only at the counter-ions outside the square brackets. If there are $n$ mono-negative counter-ions outside, the complex will dissociate into 1 complex cation and $n$ anions, making it a $1:n$ electrolyte.

Answer 1

The Correct Option is D

Concept: When a coordination compound dissolves in an aqueous solution, the chemical species inside the square brackets (the coordination sphere) remain intact as a single complex ion. The species outside the square brackets (the counter-ions in the ionization sphere) dissociate completely into individual ions. An \(x:y\) electrolyte means that upon dissociation, each formula unit produces \(x\) cation(s) and \(y\) anion(s), or vice versa, established by the respective charges of the dissociated units. Specifically, a 1:3 electrolyte releases one cation carrying a \(+3\) charge and three anions each carrying a \(-1\) charge, or one anion with a \(-3\) charge and three cations with a \(+1\) charge. Let us comprehensively analyze the dissociation behavior of each given coordination complex in an aqueous medium:

• Analysis of Option (1): \([ \text{Co}(\text{NH}_3)_3(\text{NO}_2)_3 ]\) This molecule has no counter-ions present outside the coordination sphere. Therefore, when introduced to water, it does not dissociate into separate ions. It remains as a neutral, non-electrolyte complex molecule: \[ [ \text{Co}(\text{NH}_3)_3(\text{NO}_2)_3 ] \xrightarrow{\text{H}_2\text{O}} \text{No dissociation (Non-electrolyte)} \] Total ions produced per formula unit = 0. Hence, this cannot be a 1:3 electrolyte.

• Analysis of Option (2): \([ \text{CoCl}_2(\text{NH}_3)_4 ]\text{Cl}\) This compound contains one chloride ion in its ionization sphere. Upon complete dissolution, it dissociates into one complex cation and one chloride anion: \[ [ \text{CoCl}_2(\text{NH}_3)_4 ]\text{Cl} \xrightarrow{\text{H}_2\text{O}} [ \text{CoCl}_2(\text{NH}_3)_4 ]^+ + \text{Cl}^- \] This dissociation yields a total of 2 ions in a 1:1 ratio. Thus, it represents a 1:1 electrolyte.

• Analysis of Option (3): \([ \text{CoCl}(\text{NH}_3)_5 ]\text{Cl}_2\) This complex contains two chloride counter-ions outside the coordination bracket. In an aqueous medium, it undergoes complete ionization to form one complex cation carrying a \(+2\) charge and two chloride anions: \[ [ \text{CoCl}(\text{NH}_3)_5 ]\text{Cl}_2 \xrightarrow{\text{H}_2\text{O}} [ \text{CoCl}(\text{NH}_3)_5 ]^{2+} + 2\text{Cl}^- \] This dissociation produces a total of 3 ions in a 1:2 ratio. Therefore, it acts as a 1:2 electrolyte.

• Analysis of Option (4): \([ \text{Co}(\text{NH}_3)_6 ]\text{Cl}_3\) Here, three chloride ions reside outside the coordination sphere. When dissolved in water, the compound breaks up completely to form a single hexamminecobalt(III) complex cation with a \(+3\) charge, along with three separate chloride anions: \[ [ \text{Co}(\text{NH}_3)_6 ]\text{Cl}_3 \xrightarrow{\text{H}_2\text{O}} [ \text{Co}(\text{NH}_3)_6 ]^{3+} + 3\text{Cl}^- \] This ionization releases 1 cation and 3 anions, which perfectly matches the definition of a 1:3 electrolyte.