Presence of a nitro group in a benzene ring

The pK_a of a weak acid (HA) is 4.5. The pOH of an aqueous buffered solution of HA in which 50% of the acid is ionized is :

In a sautrated solution of the sparingly soluble strong electrolyte AgIO₃ (Molecular mass = 283) the equilibrium which sets in is
AgIO_3(s) $$\leftrightharpoons$$ Ag⁺(aq) + $$IO_3^-$$
If the solubility product constant K_sp of AgIO₃ at a given temperature is 1.0 $$\times$$10⁻⁸, what is the mass of AgIO₃ contained in 100 ml of its saturated solution?

The first and second dissociation constants of an acid H₂A are 1.0 $$\times$$ 10⁻⁵ and 5.0 $$\times$$ 10⁻¹⁰ respectively. The overall dissociation constant of the acid will be :