Which of the following pairs cannot exist together in a solution?

Identify the conjugate acid-base pair in the following reaction: $HCl_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O_{(aq)}^{+} + Cl_{(aq)}^{-}$

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

For the following two acid-base reactions,which statement is True?
$(I) CH_3CH_2^- + CH_3NH_2$ $(pK_a = 35)$ $\rightleftharpoons CH_3CH_3$ $(pK_a = 50)$ $+ CH_3NH^-$
$(II) F^- + H_2O$ $(pK_a = 15.7)$ $\rightleftharpoons HF$ $(pK_a = 3.2)$ $+ HO^-$

When hydrogen chloride is dissolved in acetic acid,the following equilibrium is established: $HCl + CH_3COOH \rightleftharpoons Cl^- + CH_3COOH_2^+$. Identify the conjugate acid-base pairs.

According to the Bronsted-Lowry concept,a base is a substance which is: