$H_{2}O$ forms following conjugate base:

Question

Accepted Answer

The correct option is (4):

In acid-base chemistry, a conjugate base is formed when an acid donates a proton ($H^{+}$). Water ($H_{2}O$) is an amphoteric substance, meaning it can act as both an acid and a base. When $H_{2}O$ acts as an acid, it donates a proton. The removal of a proton ($H^{+}$) from a water molecule ($H_{2}O$) leaves behind a hydroxide ion ($OH^{-}$). This can be represented by the following equilibrium:

$\mathrm{H_2O (acid) \rightleftharpoons H^{+} + OH^{-} (conjugate \ base)}$

According to the Brønsted-Lowry theory, an acid is a proton donor and its conjugate base is the species that remains after the acid has donated its proton. Therefore, when $H_{2}O$ acts as an acid, its conjugate base is $OH^{-}$.

The incorrect option is (1): $H\_{3}O^{\oplus}$
$H\_{3}O^{\oplus}$ (hydronium ion) is formed when water acts as a base and accepts a proton. This is the conjugate acid of $H\_{2}O$, not its conjugate base.

The incorrect option is (2): $H\_{2}O$
$H\_{2}O$ is the original molecule; it has neither donated nor accepted a proton to form a conjugate species. It is acting as the acid itself in this context, not the conjugate base.

The incorrect option is (3): $H^{\oplus}$
$H^{\oplus}$ (a proton) is what an acid donates. It is not the conjugate base, but rather what is removed from the acid to form the conjugate base.