In the reaction $HBr + H_2O \rightleftharpoons H_3O^{+} + Br^{-}$,the conjugate base of the weak acid is .......

Three reactions involving $H_2PO_4^-$ are given below :
$(i) \; H_3PO_4 + H_2O \rightarrow H_3O^{+} + H_2PO_4^-$
$(ii) \; H_2PO_4^- + H_2O \rightarrow HPO_4^{2-} + H_3O^{+}$
$(iii) \; H_2PO_4^- + OH^{-} \rightarrow H_2O + HPO_4^{2-}$
In which of the above does $H_2PO_4^-$ act as an acid?

$A$ tribasic acid is:

Which of the following cannot act as a Lewis or Bronsted acid?

What is meant by the conjugate acid-base pair? Find the conjugate acid/base for the following species:
$HNO_{2}$,$CN^{-}$,$HClO_{4}$,$F^{-}$,$OH^{-}$,$CO_{3}^{2-}$,and $S^{2-}$

According to the Bronsted-Lowry theory,the acids in the following reaction are:
$ClO_4^{-} + HCO_3^{-} \rightarrow HClO_4 + CO_3^{2-}$