Question

Which of the following represents conjugate acid-base pair?

• A. ${\mathrm{H}}_3{\mathrm{O}}^{+}\mathrm{and}{\mathrm{H}}_2\mathrm{O}$
• B. ${\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{and}{{\mathrm{SO}}_4}^{2-}$
• C. ${{\mathrm{HCO}}_3}^{-}\mathrm{and}{{\mathrm{CO}}_3}^{2-}$
• D. All are conjugate acid/base pairs

Answer 1

Answer: (A), (C)

${{\mathrm{HCO}}_3}^{-}\mathrm{and}{{\mathrm{CO}}_3}^{2-}$

Explanation for the correct options:

A) ${\mathrm{H}}_3{\mathrm{O}}^{+}\mathrm{and}{\mathrm{H}}_2\mathrm{O}$

${\mathrm{H}}_3{\mathrm{O}}^{+}\mathrm{and}{\mathrm{H}}_2\mathrm{O}$ are conjugate acid/base pairs.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{OH}}^{-} \\ \left(\mathrm{Acid}\right)\left(\mathrm{Base}\right)(\mathrm{Conjugate}(\mathrm{Conjugate}) \\ \mathrm{acid})\mathrm{base}) \end{gathered}$$

From this dissociation reaction, it is clear that ${\mathrm{H}}_2\mathrm{O}$ can act as a base and ${\mathrm{H}}_3{\mathrm{O}}^{+}$is conjugate acid of the base ( ${\mathrm{H}}_2\mathrm{O}$).

C) ${{\mathrm{HCO}}_3}^{-}\mathrm{and}{{\mathrm{CO}}_3}^{2-}$

${{\mathrm{HCO}}_3}^{-}\mathrm{and}{{\mathrm{CO}}_3}^{2-}$ are conjugate acid/base pairs.

$$\begin{gathered} {{\mathrm{HCO}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}^{+}+{{\mathrm{CO}}_3}^{2-} \\ \left(\mathrm{Acid}\right)\left(\mathrm{Base}\right)(\mathrm{Conjugate}(\mathrm{Conjugate}) \\ \mathrm{acid})\mathrm{base}) \end{gathered}$$

From this dissociation reaction, it is clear that ${{\mathrm{HCO}}_3}^{-}$ act as an acid and ${{\mathrm{CO}}_3}^{2-}$is conjugate base of ${{\mathrm{HCO}}_3}^{-}$ .

Explanation for the incorrect options:

B) ${\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{and}{{\mathrm{SO}}_4}^{2-}$

${\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{and}{{\mathrm{SO}}_4}^{2-}$ are not conjugate acid/base pairs.

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{H}{{\mathrm{SO}}_4}^{-} \\ \left(\mathrm{Acid}\right)\left(\mathrm{Base}\right)(\mathrm{Conjugate}(\mathrm{Conjugate}) \\ \mathrm{acid})\mathrm{base}) \end{gathered}$$

$$\begin{gathered} {{\mathrm{HSO}}_4}^{-}+{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}^{+}+{{\mathrm{SO}}_4}^{2-} \\ \left(\mathrm{Acid}\right)\left(\mathrm{Base}\right)(\mathrm{Conjugate}(\mathrm{Conjugate}) \\ \mathrm{acid})\mathrm{base}) \end{gathered}$$

From the above dissociation equations, it is understood that ${\mathrm{H}}_2{\mathrm{SO}}_4$is an acid and its conjugate base is ${{\mathrm{HSO}}_4}^{-}$not ${{\mathrm{SO}}_4}^{2-}$. And ${{\mathrm{SO}}_4}^{2-}$ is the conjugate base of ${{\mathrm{HSO}}_4}^{-}$.

Hence, options (A) and (C) are correct.