Question

All of the following are acid-base conjugate pairs:
A) $\text{ HONO , NO}_{2}^{-}\text{ }$
B) $\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{-}}\text{ , O}{{\text{H}}^{-}}\text{ }$
C) $\text{ C}{{\text{H}}_{\text{3}}}\text{NH}_{3}^{-}\text{ , C}{{\text{H}}_{\text{3}}}\text{N}{{\text{H}}_{\text{2}}}\text{ }$
D) $\text{ HS , }{{\text{S}}_{\text{2}}}\text{ }$

Answer 1

According to the Lowry Bronsted acid-base theory, the acid is a chemical substance that donates a proton is acid, and one which accepts the proton is base. The acid forms a conjugate base and base forms a conjugate acid. The general reaction is as shown below,
$\text{ Acid + Base }\rightleftharpoons \text{ Conjugate Base + Conjugate acid }$

Complete step by step answer:
Lowry and Bronsted suggested a general definition of acid and base. According to their concept, an acid is defined as a substance that tends to donate a proton to any other substance, and a base as a substance that tends to accept a proton from any other substance.
In other words, an acid is a proton –donor, and base is a proton acceptor.
When an acid loses a proton i.e. an $\text{ }{{\text{H}}^{\text{+}}}\text{ }$ ions the residual part tends to regain the proton. This is known as the conjugate base.
When the base accepts the proton the residual part tends to lose a proton. This is known as the conjugate acid.
The general acid-base reaction and its conjugate acid and base is given as follows,
$\text{ }\begin{matrix} \text{HA} & \text{+} & \text{B} & \rightleftharpoons & {{\text{A}}^{-}} & \text{+} & \text{BH} \\\ (\text{Acid}) & {} & (\text{Base}) & {} & (\text{Conjugate Base)} & {} & (\text{Conjugate Acid)} \\\ \end{matrix}\text{ }$
Let's have a look at each option given,
According to the Lowry-Bronsted theory Consider the dissociation of nitrous acid $\text{ HONO }$in water may be represented as,
$\text{ }\begin{matrix} \text{HONO} & \text{+} & {{\text{H}}_{\text{2}}}\text{O} & \rightleftharpoons & \text{NO}_{2}^{-} & \text{+} & {{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}} \\\ \text{(Acid)} & {} & \text{(Base)} & {} & \text{(Conjugate Base)} & {} & \text{(Conjugate Acid)} \\\ \end{matrix}\text{ }$
It is evident that the nitrous acid loses its proton to the water and thus it acts as an acid. Water accepts the proton from nitrous acid and thus acts as a base. In the reverse reaction, hydronium ion $\text{ ( }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ ) }$ donates a proton to the nitrite ion $\text{ NO}_{2}^{-}\text{ }$accepts the proton, and therefore, behaves as a base. Such pairs of substances which can be formed from one another by the gain or loss of a proton are known as the conjugate acid-base pair. Thus, nitrous acid is the conjugate acid of nitrite ion and nitrite ion is the conjugate base of nitrous acid. Similarly, water is the conjugate base of hydronium ion and hydronium ion is the conjugate acid of water.
The $\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{-}}\text{ , O}{{\text{H}}^{-}}\text{ }$ , $\text{ C}{{\text{H}}_{\text{3}}}\text{NH}_{3}^{-}\text{ , C}{{\text{H}}_{\text{3}}}\text{N}{{\text{H}}_{\text{2}}}\text{ }$ and $\text{ HS , }{{\text{S}}_{\text{2}}}\text{ }$ are not the acid-base conjugate pairs.

Hence, (A) is the correct option.

Note: strong Bronsted Lowry acid is one which can easily donate the proton, then the corresponding base will be weak. Weak Bronsted Lowry acid has a less tendency to donate a proton; the corresponding conjugate base is strong in nature.