Question

Four species are listed below
i. $HC{O}_3{}^{-}$
ii. $H_3{O}^{+}$
iii. $HS{O}_4^{-}$
iv. $HS{O}_{3}F$
Which one of the following is the correct sequence of their acid strength?

• A. iv < ii < iii < i
• B. ii < iii < i < iv
• C. i < iii < ii < iv
• D. iii < i < iv < ii

Answer 1

The correct option is A iv < ii < iii < i

$HS{O}_{3}F$ is a super acid. Its acidic strength is greater than any given species. The $p{K}_a$ value of other species are

${HC{O}_3}^{-}⟶10.25$

${H_{3}O}^{+}⟶-1.74$

${HS{O}_4}^{-}⟶1.92$

lesser the $p{K}_a$ value, higher will be its acidic strength. Hence, sequence of acidic strength will be

$HS{O}_{3}F>{H_{3}O}^{+}>{HS{O}_4}^{-}>{HC{O}_3}^{-}$