$BF_3$ is an acid according to the ....... concept.

Accepting the definition that an acid is a proton donor,the acid in the following reaction $NH_3 + H_2O \to NH_4^+ + OH^-$ is

The ionization constants of two bases $AOH$ and $BOH$ are $K_{b1}$ and $K_{b2}$ respectively. Their relationship is $pK_{b1} < pK_{b2}$. Which of the following does not represent the base with the highest $pH$?

The conjugate acid of $HS^{-}$ is

The conjugate base of $H_3O^{+}$ is

The species which acts as a $Lewis$ acid but not a $Bronsted$ acid is