Which ion in excess $NaOH$ does not form a soluble complex?

Complex $A$ has a composition of $H_{12}O_{6}Cl_{3}Cr$. If the complex on treatment with conc. $H_{2}SO_{4}$ loses $13.5\%$ of its original mass,the correct molecular formula of $A$ is :
[Given : atomic mass of $Cr = 52 \ amu$ and $Cl = 35.5 \ amu$]

$CuSO_4$ solution reacts with excess of $KCN$ solution to form:

Violet colour of $[Ti(H_2O)_6]Cl_3$ on heating changes to $........$

An aqueous solution of $CoCl_2$ on addition of excess of concentrated $HCl$ turns blue due to the formation of:

$HgCl_2$ is a covalent compound,sparingly soluble in water. The solubility increases by the addition of chloride ions due to: