JEE Chemistry 2025 Question with Solution

Given: One mole of $Co(NH_3)_5Cl_3$ gives $3$ moles of ions in water and forms two moles of $AgCl$ with excess $AgNO_3$.

Find: The correct structure of the complex.

The number of ions formed in water tells us how many species are present after dissociation, and the amount of $AgCl$ formed tells us how many chloride ions are outside the coordination sphere.

For Option A, the complex is

$$[Co(NH_3)_5Cl]Cl_2$$

On dissolution in water, it gives

$$[Co(NH_3)_5Cl]^{2+} + 2Cl^-$$

So total ions $= 3$.

With excess $AgNO_3$, the two free chloride ions react as

$$2Cl^- + 2Ag^+ \rightarrow 2AgCl(s)$$

Thus, this option gives exactly two moles of $AgCl$.

Hence, this structure satisfies both given conditions.

Therefore, the correct option is A, and the structure is $[Co(NH_3)_5Cl]Cl_2$.

Given: The complex gives $3$ ions in water and yields $2$ moles of $AgCl$ with excess $AgNO_3$.

Find: Which option matches both observations.

1. Option A: $[Co(NH_3)_5Cl]Cl_2$

• One chloride is coordinated and two chlorides are free.
• Dissociation:

$$[Co(NH_3)_5Cl]Cl_2 \rightarrow [Co(NH_3)_5Cl]^{2+} + 2Cl^-$$

• Total ions $= 3$.
• Free chlorides $= 2$, so $2$ moles of $AgCl$ form.

2. Option B: $[Co(NH_3)_4Cl]\cdot Cl_2NH_3$

• This does not fit the required ionization behavior stated in the solution.

3. Option C: $[Co(NH_3)_4Cl_2]Cl\cdot NH_3$

• Here more chlorides are coordinated, so the number of free chloride ions does not support formation of $2$ moles of $AgCl$.

4. Option D: $[Co(NH_3)_3Cl]\cdot Cl_3\cdot 2NH_3$

• This gives an incorrect stoichiometric picture and does not match the required number of ions.

So only Option A satisfies both conditions.

Therefore, the correct option is A.