What is the color of a cobalt chloride solution?

Which product is formed when an excess of $KCN$ is added to a solution of copper sulfate?

$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^{+}$; $k_1 = 3.5 \times 10^{-3}$
$[Ag(NH_3)]^{+} + NH_3 \rightleftharpoons [Ag(NH_3)_2]^{+}$; $k_2 = 1.7 \times 10^{-3}$
Then the formation constant of $[Ag(NH_3)_2]^{+}$ is

If $1/3$ of the total chlorine in the compound is precipitated on adding $AgNO_3$ to its aqueous solution,which of the following is the correct structure of $CrCl_3 \cdot 6H_2O$?

The formula of the product formed, when sodium thiosulphate solution is added to silver bromide is

The brown ring test for $NO_2^-$ and $NO_3^-$ ions is due to the formation of which of the following complex ions?