Question

Which of the following is the conjugate acid of $S{O_4}^{2 - }$ $?$
A) $HS{O_4}^ - $
B) ${H^ + }$
C) ${H_2}S{O_4}$
D) $S{O_4}^{2 - }$

Answer 1

Hint: The concept of the conjugate acid is based on the Bronsted-Lowry theory. One can apply this theory concept to solve the question accurately.

Complete step by step answer:
1) The Bronsted-Lowry theory is a theory based on acid and base reaction.
2) According to Bronsted-Lowry theory, an acid is a species that has the capacity of donating a proton that is hydrogen ion ${H^ + }$
3) According to Bronsted-Lowry theory, the base is a species that has the capacity of accepting a proton that is hydrogen ion ${H^ + }$ and it needs to have a lone pair of electrons on the base which bonds to the hydrogen ion.
4) The conjugate acid of a Bronsted-Lowry base is the chemical species which is formed after the base accepts the proton that is hydrogen ion.
5) Hence, by applying this concept to find conjugate acid of $S{O_4}^{2 - }$ we get the following conjugate acid,
$S{O_4}^{2 - } + {H^ + } \to HS{O_4}^ - $
6) Therefore, the conjugate acid of $S{O_4}^{2 - }$ is $HS{O_4}^ - $ which shows the option A as a correct choice of answer.

And hence option A is a correct choice.

Note:
While the formation of a conjugate acid there is only the addition of one hydrogen should be done as the base accepts only one proton and does not go for full stability state by accepting another proton which in the case of the above question has happened. In the above reaction the $S{O_4}^{2 - }$ acts as a Bronsted-Lowry base which accepts only one proton and forms its conjugate acid as $HS{O_4}^ - $. One must not confuse that the conjugate acid form of a base will not be as a full acidic formula, there is only gain of one proton in that case.