Question

What is meant by conjugate acid – base pair? Find the conjugate acid – base for the following species.
$HN{O_2},C{N^ - },HCl{O_4}{F^ - },O{H^ - },CO_3^{2 - }$ and ${S^{2 - }}$

Answer 1

Hint:
The acid – base pairs which differ by a proton is called conjugate acid – base pairs. Conjugate acid is formed when an acid donates proton to base while conjugate base gains a proton in the chemical reaction.

Complete step by step answer:
The substance which is formed when an acid donates a proton to base in chemical reaction is called conjugate acid. In other words, we can also say that, it is the base with which the hydrogen added to it while left substance after donating the proton from acid during chemical reaction is called conjugate base. A cation can be conjugate acid and an anion can be conjugate base.
Bronsted and Martin Lowry proposed that any compound which transfers to the other compound is called acid and the compound which accepts the proton is called base. This theory is called Bronsted – Lowry theory.
The strength of conjugate acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have a higher equilibrium constant and the product of reaction will be favored. The tendency of the conjugate base to pull a hydrogen proton towards itself is called conjugate base.
$HN{O_2}(acid) \Rightarrow {H^ + } + N{O^{2 - }}$ (conjugate base)
$C{N^ - }(base) + {H^ + } \Rightarrow HCN$ (conjugate acid)
$HCl{O_4}(acid) \Rightarrow HCl{O^{3 - }} + {H^ + }$ (conjugate base)
${F^ - }(base) + {H^ + } \Rightarrow HF$ (conjugate acid)
$O{H^ - }(base) + {H^ + } \Rightarrow {H_2}O$ (conjugate acid)
$CO_3^{2 - }(base) + {H^ + } \Rightarrow HC{O_3}^ - $ (conjugate acid)
${S^{2 - }}(base) + {H^ + } \Rightarrow H{S^ - }$ (conjugate acid)


Note:
The conjugate acid – base pairs are used in buffering systems in which weak acid and its conjugate base or weak base and its conjugate acid are used in order to limit the pH change during titration. These pairs can also be used with the organic compound for production of buffers with acetic acid.