Question

The conjugate base of $H_{2}P{O}_4^{-}$ is

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

Answer 1

Answer: (D)

$H_{3}P{O}_4$ is a tribasic acid, thus ionising in three steps: I. $H_{3}P{O}_4{H}^{+}+H_{2}P{O}_4^{-}$ II. $H_{2}P{O}_4^{-}{H}^{+}+HP{O}_4^{2-}$ III. $HP{O}_4^{2-}{H}^{+}+P{O}_4^{3-}$ Conjugate base is formed when an acid loses its proton. Thus $HP{O}_4^{2-}$ is the conjugate base of $H_{2}P{O}_4^{2-}$ (which is an acid in step II, but is the conjugate base of $H_{3}P{O}_4$ in step I).