Question

Conjugate base of ${{HCl}}$ in the following reaction is:
${{HCl}} + {{{H}}_2}{{O}} \rightleftharpoons {{{H}}_3}{{{O}}^ + } + {{C}}{{{l}}^ - }$
A. ${{{H}}_3}{{{O}}^ + }$
B. ${{{H}}_2}{{O}}$
C. ${{C}}{{{l}}^ - }$
D. ${{HCl}}$

Answer 1

We know that ${{HCl}}$ is a very strong acid. Acid-base theory is based on three theories. They are Arrhenius concept, Lowry-Bronsted concept and Lewis concept. Using the Lowry-Bronsted concept, we can determine the conjugate base of ${{HCl}}$.

Complete step by step answer:
According to Arrhenius concept, a substance which dissociates in water to give ${{{H}}^ + }$ ions and bases in water give ${{O}}{{{H}}^ - }$ ions.
Now, based on the Lowry-Bronsted concept, the substances which donate protons or hydrogen ions are called acids and the substances which accept protons or hydrogen ions are called bases.
When the acids are dissolved in water, it dissociates into hydronium ions. ${{HCl}}$ is a very strong acid. The concept of conjugate acid-base pairs comes under this concept. When an acid gives a proton, it produces a conjugate base.
When ${{HCl}}$ is dissolved in water, the reaction is as given below:
${{HCl}} + {{{H}}_2}{{O}} \rightleftharpoons {{{H}}_3}{{{O}}^ + } + {{C}}{{{l}}^ - }$
In this chemical reaction, hydrochloric acid act as an acid and water act as a base. ${{HCl}}$ donates ${{{H}}^ + }$ ions to ${{{H}}_2}{{O}}$. Since ${{HCl}}$ gives ${{{H}}^ + }$ ions, it act as acid. ${{HCl}}$ produces ${{C}}{{{l}}^ - }$ ions which is the conjugate base of ${{HCl}}$. ${{{H}}_2}{{O}}$ accepts ${{{H}}^ + }$ ions to produce ${{{H}}_3}{{{O}}^ + }$. Thus ${{{H}}_2}{{O}}$ act as base and ${{{H}}_3}{{{O}}^ + }$ act as the conjugate acid of ${{{H}}_2}{{O}}$.
Hence the conjugate base of ${{HCl}}$ is ${{C}}{{{l}}^ - }$.

Hence, the correct option is C.

Note:
A conjugate acid-base pair has two substances which are connected with the loss or gain of protons. Water can, sometimes, act as both acid and base. Such compounds are called amphoteric. For example, when ${{HCl}}$ is dissolved in water, water acts as a base. When ${{N}}{{{H}}_3}$ is dissolved in water, water acts as an acid.