What is the color of the aqueous solution of $[Co(NH_3)_6]^{3+}$?

Given the following reactions:
$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^+ ; K_1 = 1.6 \times 10^3$
$[Ag(NH_3)]^+ + NH_3 \rightleftharpoons [Ag(NH_3)_2]^+ ; K_2 = 6.8 \times 10^3$
What is the formation constant for $[Ag(NH_3)_2]^+$?

When aqueous sodium hydroxide is added to an aqueous solution of chromium $(III)$ ions,a green-blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to

Which among the following is a correct decreasing order of thermodynamic stability of the complexes?

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6 NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in solution were treated with excess of $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is
(At. mass of $Ag = 108 \ u$)

$PbCl_{2}$ is insoluble in cold water. Addition of $HCl$ increases its solubility due to