What is the conjugate base of $OH^{-}$?

The enthalpies of neutralization of $HCN$ and $H_2S$ with a strong base are $12.13 \ kJ \ equi^{-1}$ and $15.9 \ kJ \ equi^{-1}$ respectively. Then:

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

On dissolving $1 \ mole$ of each of the following acids in $1 \ L$ of water,the acid which does not give a solution of strength $1 \ N$ is:

The concentration of which ion is decreased when $NH_3$ solution is added?

Which of the following cannot act both as Bronsted acid and as Bronsted base?