Question

What is the conjugate base of $HS{O_4}^-$ ?

Answer 1

Conjugate base is the substance formed when an acid loses a hydrogen ion. Considered a base because it gains a hydrogen ion to reform the acid. It can be represented as:
Acid $+$ Base $\rightleftharpoons$ Conjugate Base $+$ Conjugate Acid.
A cation can be a conjugate acid and anion can be a conjugate base.

Complete answer:
The Bronsted-Lowrey theory state that, when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (The hydrogen cation, or ${H^ + }$ ).
Example- $HCl + {H_2}O \to {H_3}O + C{l^ - }$ water acts as a base (accepting ${H^ + }$ ) because $HCl$ is a stronger acid.
But in reaction, $N{H_3} + {H_2}O \to N{H_4} + O{H^ - }$ water acts as an acid (donates ${H^ + }$ to ammonia) because water is a stronger acid than ammonia.
$HS{O_4}^ -$ Is in fact the conjugate base of the sulfuric acid. ${H_2}S{O_4}$ is an acid so it donates a proton and after donating a proton it will be $HS{O_4}^ -$ . The reaction is
${H_2}S{O_4} \to HS{O_4}^ - + {H^ + }$ . $HS{O_4}^ -$ is a base since it has the ability to accept a proton but it is a conjugate base to ${H_2}S{O_4}$ since it is formed by the ${H_2}S{O_4}$ after donating a proton.
But, $HS{O_4}^ -$ is an acid itself. When in water, it removes the ${H^ + }$ proton, it becomes a conjugate base $S{O_4}^{2 - }$ . The conjugate base means it has to add ${H^ + }$ proton back in a backward reaction.
The reaction in water: $HS{O_4}^ - + {H_2}O \to S{O_4}^{2 - } + {H_3}{O^ + }$ . Even if $HS{O_4}^ -$ has a negative charge, it will never accept a ${H^ + }$ ion to form ${H_2}S{O_4}$ again. That is because sulfuric acid is a stronger acid and completely dissociates in water. That’s why sulphate ions would be the conjugate base formed.
$HS{O_4}^ -$ Is acid and $S{O_4}^{2 - }$ is conjugate base
${H_2}O$ is base and ${H_3}{O^ + }$ is conjugate acid.

Note:
The conjugate acid of a base is the compound formed after the Bronsted base accepts a proton.
$S{O_4}^{2 - } + {H^ + } \to HS{O_4}^ -$ , that is bisulfate ion $HS{O_4}^ -$ is conjugate acid of sulfate ion. (Bronsted base: a proton acceptor.) It is not possible to put this ion $HS{O_4}^ -$ into solution as it has to accompany something like sodium hydrogen sulfate $(NaHS{O_4})$ or calcium sulfate $(CaHS{O_4})$ .