One mole of the octahedral complex compound Co(NH₃)₅ Cl₃ gives 3 moles of ions on dissolution in water. One mole of the same complex reacts with excess of AgNO₃ solution to yield two moles of AgCl₍ₛ₎. The structure of the complex is:

Solution

Chemistry ma’am · 3 steps

Only chloride outside the bracket reaches the silver, so two AgCl means two ionisable chlorides.

1. Read the clues. Ag⁺ precipitates only free Cl⁻ ions, so 2 mol AgCl means 2 chlorides sit outside the coordination sphere. Those 2 Cl⁻ plus the complex ion make 3 ions, which matches the dissolution data.
2. Use octahedral. Octahedral means 6 ligands on cobalt. The one remaining chloride and all 5 NH₃ must be inside the bracket: 5 + 1 = 6.

   Option	Ligands on Co	Ions in water	Mol AgCl
   (a) [Co(NH₃)₅Cl]Cl₂	6, octahedral	3	2
   (b) [Co(NH₃)₄Cl]·Cl₂·NH₃	5, not octahedral	3	2
   (c) [Co(NH₃)₃Cl₃]· 2NH₃	6	0 (neutral)	0
   (d) [Co(NH₃)₄Cl₂]·Cl·NH₃	6	2	1

3. So. Only (a) has 6 ligands, 3 ions and 2 free chlorides, so it ionises as [Co(NH₃)₅Cl]²⁺ + 2 Cl⁻. The answer is (a).

The idea to keep. Moles of AgCl count the chlorides outside the bracket. The coordination number tells you how many ligands must stay inside. Check both before you pick a formula.

Answer from the NTA final answer key. Solution by PrepWiser.