Question

Which one of the following species acts as both Bronsted acid and base ?

• A. $H_{2}PO^{-}_{2}$
• B. $HPO^{2-}_{3}$
• C. $HPO^{2-}_{4}$
• D. All of these

Answer 1

Answer: (C)

$HPO^{2-}_{4}$

Answer 2

$HPO _{4}^{2-}+ H _{2} O \rightleftharpoons H _{2} PO _{4}^{2-}+ OH$
$HPO _{4}^{2-}+ H _{2} O \rightleftharpoons PO _{4}^{3-}+ H _{3} O ^{+}$
$H _{2} PO _{2}^{-}$ is a conjugate base of $H _{3} PO _{2}$

( a monobasic acid) and does not give $H ^{+}, HPO _{3}^{2-}$ is a conjugate base of $H _{2} PO _{3}^{-}$ and does not ionise further, since $H _{3} PO _{3}$ is a dibasic acid.