Question

Identify the species which can give both conjugate acid and conjugate base:

• A. ${NH}_4^{+}$
• B. $H_2{PO}_4^{-}$
• C. ${PO}_4^{3-}$
• D. $H_3{O}^{+}$

Answer 1

The correct option is B $H_2{PO}_4^{-}$

$N{H}_4^{+}$ and $H_3{O}^{+}$ can act as only conjugate acids as they can only donate a proton.
$P{O}_4^{3-}$ can act as only conjugate base as it can only accept a proton but $H_{2}P{O}_4^{-}$ can act as both conjugate acid and conjugate base, as it can accept a proton to form $H_{3}P{O}_4$ and can donate a proton to form $HP{O}_4^{2-}$ and $P{O}_4^{2-}$.