(3)

A complex salt is defined as a compound composed of a central metal atom having coordination bonds with ligands around it. While, double salts are defined as the ionic compounds that are formed by the combination of two different salt compounds.  

Complex salt is ${\mathrm{K}}_2\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_2\right]$.

Double salt is $\mathrm{KAl}{\left({\mathrm{SO}}_4\right)}_2·12{\mathrm{H}}_2\mathrm{O} \text{(potash alum).}$

(2)

$\left[{\text{Co(NH}}_3{\text{)}}_6\right]{\text{Cl}}_3+3{\text{AgNO}}_3 \to 3\text{AgCl}\downarrow +\left[{\text{Co(NH}}_3{\text{)}}_6\right]{({\text{NO}}_3\text{)}}_3$

$\left[{\text{Co(NH}}_3{\text{)}}_5\text{Cl}\right]{\text{Cl}}_2+2{\text{AgNO}}_3 \to 2\text{AgCl}\downarrow +\left[{\text{Co(NH}}_3{\text{)}}_5\text{Cl}\right]{({\text{NO}}_3\text{)}}_2$

$\left[{\text{Co(NH}}_3{\text{)}}_4{\text{Cl}}_2\right]\text{Cl}+{\text{AgNO}}_3 \to \text{AgCl}\downarrow +\left[{\text{Co(NH}}_3{\text{)}}_4{\text{Cl}}_2\right]{\text{NO}}_3$

(3)

For octahedral complexes, coordination number is 6. Hence, ${\text{CoCl}}_3·3{\text{NH}}_3$, i.e., $\left[{\text{Co(NH}}_3{\text{)}}_3{\text{Cl}}_3\right]$ will not ionise and will not give test for ${\mathrm{Cl}}^{-}$ ion with silver nitrate.