Which of the following is not a conjugate acid-base pair?

Which of the following is not a conjugate acid-base pair?
  1. $\mathrm{HPO}_3^{2-}, \mathrm{PO}_3^{3-}$
  2. $\mathrm{H}_2 \mathrm{PO}_4^{-}, \mathrm{HPO}_4^{2-}$
  3. $\mathrm{H}_2 \mathrm{PO}_4^{-}, \mathrm{H}_3 \mathrm{PO}_4$
  4. $\mathrm{H}_2 \mathrm{PO}_4^{-}, \mathrm{PO}_3^{3-}$

Solution

$\begin{aligned} & \text { Acid }_1+\text { Base }_2 ightleftharpoons \text { Acid }_2+\text { Base }_1 \\ & \mathrm{HPO}_3^{2-}+\mathrm{H}_2 \mathrm{O} ightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{PO}_3^{3-} \\ & \mathrm{H}_2 \mathrm{PO}_4^{-}+\mathrm{H}_2 \mathrm{O} ightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{HPO}_4^{2-} \\ & \mathrm{H}_3 \mathrm{PO}_4+\mathrm{H}_2 \mathrm{O} ightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{H}_2 \mathrm{PO}_4^{-}\end{aligned}$ .

Asked in: JEE-TOPICTESTS-CHEMISTRY

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