Question

# The conjugate base of ${\text{HSO}}_4^ -$ in aqueous solution is:A) ${\text{SO}}_4^{ - 2}$B) ${{\text{H}}_3}{\text{S}}{{\text{O}}_4}$C) ${{\text{H}}_2}{\text{S}}{{\text{O}}_4}$D) None of the above.

Hint: During dissociation, the molecules that are formed when the acids lose a hydrogen atom are called a conjugate base for that particular acid.

We know that, if we have an acid HA, it undergoes dissociation as –

$\text{HA }\text{ }\to{{\text{H}}^{+}}\text{ + }{{\text{A}}^{-}}$.

So, we call ${{\text{A}}^ - }$ to be the conjugate base of the acid HA.
If we look at ${\text{HSO}}_4^ -$, it will dissociate as –

$\text{HSO}_{4}^{-}\text{ }\to\text{ }{{\text{H}}^{+}}\text{ + SO}_{4}^{-2}$

Hence, ${\text{SO}}_4^{ - 2}$ will be the conjugate base of ${\text{HSO}}_4^ -$ in an aqueous solution.

Therefore, option A is correct.

Additional Information Lowry and Bronsted gave the concept of conjugate acid and conjugate base. It is actually an extended version of Arrhenius theory of acids and bases. The theory states that when an acid and a base react with each other, exchange of protons occurs between them and in that exchange the substance or species which donates an ion or a proton is an acid and forms its conjugate base. Whereas, the species which accepts an ion or proton is a base and it forms its conjugate acid.

$\text{Acid }\to\text{ Proton + Conjugate base}$

If the conjugate base accepts a proton, acid is reformed.

$\text{Base + proton}\to\text{ Conjugate acid}$

If the conjugate acid donates a proton, the base is reformed.

Note: Strong acid forms a weak conjugate base and weak acid forms a strong conjugate base. They are also called acid-base pairs. Substances which have strong ability to exchange protons are called as strong Lowry-Bronsted acid/base and substances which have weak ability for exchange of protons are called weak Lowry-Bronsted acid/base.