Question

Among the following, the correct order of acidity is

• A. $HClO<HCl{O}_2<HCl{O}_3<HCl{O}_4$
• B. $HCl{O}_2<HClO<HCl{O}_3<HClO$
• C. $HCl{O}_4<HCl{O}_2<HClO<HCl{O}_3$
• D. $HCl{O}_3<HCI{O}_4<HCl{O}_2<HClO$

Answer 1

The correct option is A

$HClO<HCl{O}_2<HCl{O}_3<HCl{O}_4$

The strongest acid will have the least basic conjugate base. If we look at the perchlorate anion, the electronic charge is rather well distributed due to resonance:

Hence, of all possible conjugate bases, this one is the most stable and least basic (because the charge is well distributed). Thus perchloric acid is the most acidic. As the number of oxygen atoms attached to the central chlorine atom (among the given acids) decreases, the contributing structures of the corresponding conjugate bases also decreases. Hence the order of acidity is:

$HClO<HCl{O}_2<HCl{O}_3<HCl{O}_4$