Question

Which among the following is the strongest acid?
(A) $HClO$
(B) $HCl{O_2}$
(C) $HCl{O_3}$
(D) $HCl{O_4}$

Answer 1

The given compounds are the oxy-acids of halogen. The acidic strength of a compound is the tendency of acid to dissociate into the cationic anionic form. Acidic strength of the acids depends on several factors such as electronegativity and the oxidation number of the atom.

Complete step by step answer
We can observe that the compounds given in the option are oxy-acids of the halogen atom. There are many factors and reasons which can explain the acidic strength of oxy-acids. We will compare the acidic strength of all the given oxy-acids of chlorine. So the given oxy-acids of halogens are as follows, $HClO$ is the hypochlorous acid, $HCl{O_2}$ is the chlorous acid, $HCl{O_3}$ is the chloric acid and the last one $HCl{O_4}$ is the perchloric acid. We can observe that in all the four possible oxy-acids of chlorine, $Cl$ is $s{p^3}$ hybridized.
Now we will discuss the reason for the acidic strength of oxy-acids of halogen. The acidic strength of oxy-acids of halogen can be explained based on the relative stability of anions left after removal of the proton. Now we know that the greater the number of the oxygen atom in the anion, the greater will be the dispersal of the negative charge through $p\pi - d\pi$ basic bonding, and hence greater will be the stability of the anion. Since the stability of anions decreases in the order $ClO_4^ - > ClO_3^ - > ClO_2^ - > Cl{O^ - }$ . Therefore, the acidic strength also decreases in the same order: $HCl{O_4} > HCl{O_3} > HCl{O_2} > HClO$ . The strongest acid is $HCl{O_4}$ .
Therefore, the correct option is (D).

Note:
The Acidic strength of the oxy-acids of the same halogen atom increases with the increase in the oxidation number of the halogen atom. So, among the following the oxidation number of chlorine is maximum in $HCl{O_4}$ which is $+ 7$ . Therefore, $HCl{O_4}$ is the strongest acid.