Conjugate acid of $HPO_4^{2-}$ is

Which one of the following ionic species has the greatest proton affinity to form a stable compound?

The $pK_a$ values in $H_2O$ of picric acid,acetic acid,and phenol are in the order:

Classify the following species as Lewis acids or Lewis bases: $CO_2$,$BCl_3$,$NH_3$,$CH_3NH_2$,$NO_2^+$,and $C_6H_5NH_3^+$.

The dissociation constant of a weak acid is $1.0 \times 10^{-5}$. The equilibrium constant for its reaction with a strong base is:

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A :$ The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason $R :$ Water acts as an acid with $NH_{3}$ and as a base with $H_{2}S$.
In the light of the above statements,choose the correct answer from the options given below: