Question

What is the conjugate acid of $O{H^ - }\,?$

Answer 1

Hint : The Bronsted-Lowry acid base theory defines acid as a chemical species that can donate a proton $({H^ + })$ while a base can accept a proton $({H^ + })$ . Acids and bases react to form a conjugate acid – base pair in the chemical reaction. These are formed in the product side.

Complete Step By Step Answer:
The Bronsted- Lowry theory also known as proton theory of acids and bases describes an acid as a proton donor and base as a proton acceptor. A chemical species can act as an acid only in the presence of a base and vice versa. When they react , they form a conjugate acid base pair in the product side of the chemical equation.
A conjugate acid of a base is formed when base gains a proton. Now, it acts as an acid being a potent proton donor.
A conjugate base of an acid is formed when an acid loses a proton. Once it loses the proton, it can act as a base. It can then accept a proton and come back to its original form.
Acid $+$ Base $\rightleftharpoons$ Conjugate acid $+$ Conjugate base
The above scenario can be explained with the help of following example:
$NH_4^ + + O{H^ - } \rightleftharpoons N{H_3} + {H_2}O$
Here, $NH_4^ +$ $=$ Acid
$O{H^ - }$ $=$ Base
$N{H_3}$ $=$ Conjugate base
${H_2}O$ $=$ Conjugate acid
The ammonium ion (acid) loses a proton and becomes ammonia which is the conjugate base of the ammonium ion. It is a base as it can accept a proton. Whereas the hydroxide ion (base) accepts a proton from ammonium ion and forms water.
Hence, water is the conjugate acid of $O{H^ - }$ . Water can release a proton, hence acting as a proton donor.

Note :
The terms acid, base, conjugate acid , conjugate base are not fixed, they can change according to the reaction conditions.
Water can undergo self ionisation where ${H_2}0$ molecule can itself act as acid and base and it releases and accepts protons to form conjugate base and acid respectively.
${H_2}O + {H_2}0 \rightleftharpoons O{H^ - } + {H_3}{O^ + }$
acid base conj. base conj.acid