Which compound is formed when an excess of $KCN$ is added to an aqueous solution of copper sulphate?

Compound $PdCl_4 \cdot 6H_2O$ is a hydrated complex; $1 \ m$ aqueous solution of it has a freezing point of $269.28 \ K$. Assuming $100\%$ ionization of the complex,calculate the molecular formula of the complex ($K_f$ for water $= 1.86 \ K \ kg \ mol^{-1}$,freezing point of pure water $= 273 \ K$).

The reaction of $Pb(NO_3)_2$ and $NaCl$ in water produces a precipitate that dissolves upon the addition of $HCl$ of appropriate concentration. The dissolution of the precipitate is due to the formation of

An aqueous pink solution of Cobalt $(II)$ chloride changes to deep blue on addition of excess of $HCl$. This is because:

Which of the following complexes has the highest stability at $298 \ K$?

Which complex is formed when an aqueous solution of $NH_3$ is added to a $CuSO_4$ solution,resulting in a deep blue color?