The coordination number of chromium in $K[Cr(H_2O)_2(C_2O_4)_2]$ is

Explain double salts and coordination compounds.

The oxidation number of iron in the compound formed during the brown ring test for $NO_3^{-}$ ion is . . . . . .

The coordination number of aluminum in its common complexes is

$A$ $1:3$ electrolyte in an aqueous solution is

$3$ moles of a metal complex with the formula $Co(en)_2 Cl_3$ gives $3$ moles of silver chloride on treatment with an excess of silver nitrate. The secondary valency of $Co$ in the complex is $.....$ (Round off to the nearest integer).