A Solution containing $2.675 \mathrm{g}$ of ${\mathrm{CoCl}}_3 .6{\mathrm{NH}}_3$ (molar mass $= 267.5 {\mathrm{mol}}^{-1}$ ) is passed through a cation exchanger. The chloride ions obtained in solution were treated with excess of ${\mathrm{AgNO}}_3$ to give $4.78 \mathrm{g}$of $\mathrm{AgCI}$ (molar mass $= 143.5 \mathrm{g} \mathrm{mo}{1}^{-1}$ ) . The formula of the complex is (At. mass of $\mathrm{Ag}=108 \mathrm{u}$)

Moles of ${\mathrm{CoCl}}_3\cdot 6{\mathrm{NH}}_3=\frac{2.675}{267.5}=\frac{1}{100}\text{ mole }=0.01 \mathrm{mole}$

Moles of $\mathrm{AgCl}=\frac{4.78}{143.5}=\frac{3}{100}\text{ mole }=0.03\text{ mole }$

$0.01$mole of compound gives moles of $\mathrm{AgCI} = 0.03 \mathrm{mole}$ 

So, $1$ mole of compound gives moles of $\mathrm{AgCl}=\frac{0.03}{0.01}=3 \mathrm{mole},$So, structural formula of 

compound having $3{\mathrm{Cl}}^{-}$ ions out side of coordination sphere, so formula is $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_3$