Question

Consider various species generated when $H_{3}P{O}_4$ is dissolved in water. According to the Bronsted Lowry’s Concept among these, the conjugate acid of $HP{O}_4^{2-}$ is :

• A. $HP{O}_4^{2-}$
• B. $H_{2}P{O}_4^{-}$
• C. $P{O}_4^{3-}$
• D. $H_3{O}^{+}$

Answer 1

The correct option is B $H_{2}P{O}_4^{-}$

When a Bronsted-lowry acid is in equilibrium, it will donate a proton (as acids do) in the form of a $H^{+}$, and therefore become a base (willing to accept a proton). This resulting base is called the conjugate base. Again when in equilibrium, it will accept a proton, and become a conjugate acid.
Here is the equilibrium expression:
$HP{O}_4^{2-}+H_{2}O\rightleftharpoons H_{2}P{O}_4^{-}\left(\text{conjugate acid}\right)+O{H}^{-}$.
Simply the conjugate acid of any species is that species plus a proton; so it's $H_{2}P{O}_4^{-}$.