Question

The conjugate base of $H_2{PO}_4^{-}$ is:

• A. $H{PO}_4^{2-}$
• B. $P_2{O}_5$
• C. $H_3{PO}_4$
• D. ${PO}_4^{3-}$

Answer 1

The correct option is A $H{PO}_4^{2-}$

The concept of conjugate Acid-Base pair was given by Lowry-Bronsted Theory.

According to the theory:
Acids are proton $\left(H^{+}\right)$ donors.
Bases are proton $\left(H^{+}\right)$ acceptors.

Hence, in order to get the conjugate base of a species, we should consider that substance as an acid and vice versa.

For example,

$H_{2}S{O}_4\to HS{O}_4^{-}+H^{+}$

Therefore, conjugate base of $H_{2}S{O}_4$ is $HS{O}_4^{-}$.

Using this logic, for the given example:
$H_{2}P{O}_4^{-}\to HP{O}_4^{-2}+H^{+}$

Hence, conjugate base of $H_{2}P{O}_4^{-}$ is $HP{O}_4^{-2}$.

Option A is correct.