Which of the following represents the correct order of increasing `pK_a` values of the given acids?

Options

A. `HClO_4lt HNO_3 lt H_2CO_3 lt B(OH)_3`
B. `HNO_3 lt HClO_4 lt B(OH)_3 lt H_2CO_3`
C. `B(OH)_3 lt H_2CO_3 lt HClO_4 lt HNO_3`
D. `HClO_4 lt HNO_3 lt B(OH)_3 lt H_2CO_3`

Answer

A

Explanation

More the `pK_a` value, weaker is the acid. `HClO_4` is a stronger acid than `HNO_3` (almost same electronegativity of N and Cl, but higher oxidation state of Cl In `HClO_4` than N in `HNO_3`). Both `H_2 CO_3 and B(OH)_3` are very weak acids. `CO_3^(2-)` (from `H_2CO_3`) is resonance stabilized while no such resonance is possible in `[B(OH)_4]^(-)` (from `H_3BO_3`)
`H_2CO_3 (aq) hArr 2H^(+) (aq) + CO_3^(2-)(aq)`
`B(OH)_3(aq) +H_2O (l) hArr [B(OH)_4]^(-)(aq) + H^(+)(aq)`
Thus, correct order of acid strength is `B(OH)_3 lt H_2CO_3 lt HNO_3 lt HClO_4`
Order of increasing `pK_a` values
`B(OH)_3 gt H_2CO_3 gt HNO_3 gt HClO_4`