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Three reactions involving $\mathrm{H}_2 \mathrm{PO}_4^{-}$are given below :
(i) $\mathrm{H}_3 \mathrm{PO}_4+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{H}_2 \mathrm{PO}_4^{-}$
(ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{HPO}_4^{2-}+\mathrm{H}_3 \mathrm{O}^{+}$
(iii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_3 \mathrm{PO}_4+\mathrm{O}^{2-}$
In which of the above does $\mathrm{H}_2 \mathrm{PO}_4^{-}$act as an acid ?
Options:
(i) $\mathrm{H}_3 \mathrm{PO}_4+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{H}_2 \mathrm{PO}_4^{-}$
(ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{HPO}_4^{2-}+\mathrm{H}_3 \mathrm{O}^{+}$
(iii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_3 \mathrm{PO}_4+\mathrm{O}^{2-}$
In which of the above does $\mathrm{H}_2 \mathrm{PO}_4^{-}$act as an acid ?
Solution:
2800 Upvotes
Verified Answer
The correct answer is:
(ii) only
(ii) only
(i) $\mathrm{H}_3 \mathrm{PO}_4+\mathrm{H}_2 \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\underset{\text { acriugate base }}{\mathrm{H}_2 \mathrm{PO}_4^{-}}$
(ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \underset{\text { aconiugate base }}{\mathrm{HPO}_4^{-2}}+\mathrm{H}_3 \mathrm{O}^{+}$
(iii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\underset{\text { acid }}{\mathrm{OH}^{-}} \rightarrow \underset{\text { conjugate acid }}{\mathrm{H}_3 \mathrm{PO}_4}+\mathrm{O}^{-2}$
Only in reaction (ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}$acids as 'acid'.
(ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \underset{\text { aconiugate base }}{\mathrm{HPO}_4^{-2}}+\mathrm{H}_3 \mathrm{O}^{+}$
(iii) $\mathrm{H}_2 \mathrm{PO}_4^{-}+\underset{\text { acid }}{\mathrm{OH}^{-}} \rightarrow \underset{\text { conjugate acid }}{\mathrm{H}_3 \mathrm{PO}_4}+\mathrm{O}^{-2}$
Only in reaction (ii) $\mathrm{H}_2 \mathrm{PO}_4^{-}$acids as 'acid'.
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