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

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 $C_{6}H_{5}NH_{3}^{+}$?

Aqueous ammonia is used instead of aqueous $NaOH$ solution to precipitate $Al^{3+}$ ions as $Al(OH)_3$ because .................

Which of the following can act as an acid and as a base?

The $pK_a$ for acid $A$ is greater than $pK_a$ for acid $B$. The strong acid is

The conjugate base of $NH_4^{+}$ is