On treatment of 100   mL of 0 . 1   M solution of CoCl 3 . 6 H 2 O with excess AgNO 3 ;   1 …

Moles of ${\mathrm{CoCl}}_3.6{\mathrm{H}}_2\mathrm{O}$$=100\times 0.1\times {10}^{-3}={10}^{-2}\text{mole}$

Moles of ${\mathrm{Cl}}^{-}$ ions precipitated$=\frac{1.2\times {10}^{22}}{6\times {10}^{23}}=2\times {10}^{-2} \text{mole}$

So, one mole of complex will release two moles of chloride ions so, formula of complex is 

$\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{Cl}\right]{\mathrm{Cl}}_2.\mathrm{ }{\mathrm{H}}_2\mathrm{O}$