Identify conjugate acid and conjugate base for $\mathrm{HCO}_3^{-}$ion respectively

Find the concentration of sodium acetate when added to 0.1 M solution of acetic acid to form a buffer solution of $\mathrm{pH}=5.5$ ?

( $\mathrm{pK}_{\mathrm{a}}$ of $\mathrm{CH}_3 \mathrm{COOH}=4.5$ )

Calculate the ionisation constant of $0.08 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ of a monobasic acid having $\mathrm{pH}=2$.

The solubility product of salt $\mathrm{B}_2 \mathrm{~A}$ is $3.2 \times 10^{-11}$ at 298 K . What is solubility of the salt at same temperature?