According to the Lewis concept,which one of the following is not a base?

What is meant by the conjugate acid-base pair? Find the conjugate acid/base for the following species:
$HNO_{2}$,$CN^{-}$,$HClO_{4}$,$F^{-}$,$OH^{-}$,$CO_{3}^{2-}$,and $S^{2-}$

If $pK_b$ for fluoride ion $(F^-)$ at $25\,^{\circ}C$ is $10.83$,the ionisation constant $(K_a)$ of hydrofluoric acid $(HF)$ in water at this temperature is:

Which one of the following statements is not correct?

The $K_{a}$ values of $A$,$B$,and $C$ are $1.8 \times 10^{-4}$,$5 \times 10^{-10}$,and $3 \times 10^{-8}$ respectively. The correct order of their acidic strength is

Which salt can be classified as an acid salt?