Conjugate acid of $NH_2^{-}$ and $NH_3$ are respectively

Explain the existence of hydronium ion in aqueous solution.

The $pK_a$ of a weak acid is defined as:

How many of the following are diprotic acids? Citric acid, Chromic acid, Oxalic acid, Pyrosulfuric acid, Sulfurous acid

Which of the following species are only Lewis acids but not Bronsted acids from $CO_{2}$,$BCl_{3}$,$NH_{3}$,$CH_{3}NH_{2}$,$NO_{2}^{+}$,and $C_{6}H_{5}NH_{3}^{+}$?

Identify the strongest base according to the Bronsted-Lowry theory from the following.