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Which one of the following species acts as both Bronsted acid and base?
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Verified Answer
The correct answer is:
$\mathrm{HPO}_{4}^{2-}$
$\mathrm{HPO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{2} \mathrm{PO}_{4}^{2-}+\mathrm{OH}^{-}$
$\mathrm{HPO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{PO}_{4}^{3-}+\mathrm{H}_{3} \mathrm{O}^{+}$
$\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ is a conjugate base of $\mathrm{H}_{3} \mathrm{PO}_{2}$ (a
monobasic acid) and does not give $\mathrm{H}^{+}, \mathrm{HPO}_{3}^{2-}$ is a conjugate base of $\mathrm{H}_{2} \mathrm{PO}_{3}^{-}$ and does not ionise further, since $\mathrm{H}_{3} \mathrm{PO}_{3}$ is a dibasic acid.
$\mathrm{HPO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{PO}_{4}^{3-}+\mathrm{H}_{3} \mathrm{O}^{+}$
$\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ is a conjugate base of $\mathrm{H}_{3} \mathrm{PO}_{2}$ (a
monobasic acid) and does not give $\mathrm{H}^{+}, \mathrm{HPO}_{3}^{2-}$ is a conjugate base of $\mathrm{H}_{2} \mathrm{PO}_{3}^{-}$ and does not ionise further, since $\mathrm{H}_{3} \mathrm{PO}_{3}$ is a dibasic acid.
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