Question

What is the conjugate base of $ {H_2}O $ ?

Answer 1

Hint :Use the Bronsted Lowry acid base theory definitions of an acid, base and their conjugate acid and conjugate base to find the conjugate base of $ {H_2}O $ . Water undergoes disproportionation to give a proton and a hydronium ion.

Complete Step By Step Answer:
In terms of proton transfer between chemical species, the Bronsted-Lowry theory explains acid-base interactions. Any species that can release a proton is a Bronsted-Lowry acid, and any species that can accept a proton is a base. A Bronsted-Lowry base must have at least one lone pair of electrons to accept a proton and form a new bond with it.
An acid-base reaction, according to the Bronsted-Lowry theory, is any reaction in which a proton is transferred from an acid to a base. The Bronsted-Lowry concepts can be used to talk about acid-base reactions in any solvent, even those that happen in the gas phase.
The species formed after the base accepts a proton in a Bronsted-Lowry acid-base reaction is a conjugate acid. A conjugate base, on the other hand, is the result of an acid donating its proton. The molecular formulas of the two species in a conjugate acid-base pair are identical, except that the acid has an extra $ {H^ + } $ compared to the conjugate base.
Now, let us consider the auto-ionization of water.
 $ {{\text{H}}_2}{\text{O + }}{{\text{H}}_2}{\text{O}} \to {{\text{H}}_3}{{\text{O}}^ + } + {\text{O}}{{\text{H}}^ - } $
Here, two water molecules undergo disproportionation reactions and result in a transfer of proton to form a hydronium ion and a hydroxide ion. So, this is a Bronsted-Lowry acid-base reaction. Here, the conjugate base is the compound formed after losing a proton which is the $ {\text{O}}{{\text{H}}^ - } $ ion. Hence, the conjugate base of $ {H_2}O $ is $ {\text{O}}{{\text{H}}^ - } $ .

Note :
Note that in case of acids which are capable of releasing multiple protons, the conjugate base of an acid will also be an acid. Don’t get confused by the terminology used. To find the conjugate base of an acid simply remove a $ {H^ + } $ from its formula. For example, the conjugate base of $ {H_2}S{O_4} $ is $ HSO_4^ - $ .