An octahedral complex with the formula CoCl₃·nNH₃ upon reaction with excess of AgNO₃ solution gives 2 moles of AgCl.…

🧠 Precipitation Tells You Outer-Sphere Cl⁻

2 mol AgCl per 1 mol complex → 2 Cl⁻ are ionised (outer sphere). The remaining 1 Cl⁻ stays inside the coordination sphere.

🗺️ Deduce Formula

Total Cl: 3. Outer: 2. Inner: 1. Plus n NH₃ in inner sphere. Formula: $[\mathrm{Co(NH_3)_n Cl}]\mathrm{Cl_2}$.

For octahedral coordination, inner-sphere donors must total 6: $n + 1 = 6 \Rightarrow n = 5$.

🗺️ Cobalt Oxidation State

Charge balance on the complex cation $[\mathrm{Co(NH_3)_5 Cl}]^{2+}$: $x + 5(0) + (-1) = +2 \Rightarrow x = +3$.

So $x = 3$, $n = 5$.

$x + n = 3 + 5 = 8$

⚡ Werner's Rule of Thumb

Number of moles of AgCl precipitated = number of ionisable (outer-sphere) Cl⁻. JEE asks this every cycle. Memorise:

| Complex | AgCl ppt | Inner-sphere Cl⁻ | |---|---|---| | $\mathrm{CoCl_3 \cdot 6NH_3}$ | 3 | 0 | | $\mathrm{CoCl_3 \cdot 5NH_3}$ | 2 | 1 | | $\mathrm{CoCl_3 \cdot 4NH_3}$ | 1 | 2 | | $\mathrm{CoCl_3 \cdot 3NH_3}$ | 0 | 3 |

⚠️ Co Almost Always +3 with NH₃

Co(III) is the textbook stable OS in ammine complexes (especially with chloride co-ligands). Don't second-guess: charge balance always confirms it.

$\boxed{\text{Answer: (2) 8}}$