Which is conjugate base of $OH^{-1}$?

Question

Accepted Answer

The correct option is (1):

To understand why $O^{-2}$ is the conjugate base of $OH^{-1}$, we need to recall the Brønsted-Lowry acid-base theory. This theory defines an acid as a proton ($H^+$) donor and a base as a proton ($H^+$) acceptor. When an acid donates a proton, it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid. In this question, we are looking for the conjugate base of $OH^{-1}$. This means that $OH^{-1}$ must be acting as a Brønsted-Lowry acid, donating a proton. When $OH^{-1}$ donates one $H^+$ ion, it loses both the hydrogen atom and a positive charge, resulting in an $O^{-2}$ ion. Therefore, $O^{-2}$ is the species formed after $OH^{-1}$ loses a proton, making it the conjugate base.

The incorrect option is (2):

$H_2O$ is the conjugate acid of $OH^{-1}$. If $OH^{-1}$ were to act as a base and accept a proton ($H^+$), it would form $H_2O$. This relationship demonstrates how $OH^{-1}$ can function as a base, rather than an acid, in a different context.

The incorrect option is (3):

$H_3O^+$ is the hydronium ion, which is typically formed when an acid donates a proton to a water molecule. It is the conjugate acid of $H_2O$. $H_3O^+$ represents a highly acidic species and is not directly related to $OH^{-1}$ as a conjugate base.

The incorrect option is (4):

"All of the above" is incorrect because only one of the options can be the true conjugate base of $OH^{-1}$ according to the Brønsted-Lowry theory. As explained, $O^{-2}$ is the only species formed when $OH^{-1}$ loses a proton.