Question

Using your knowledge of the molecular basis for acid strength, explain the following observations:
i) HClO4 is a stronger oxyacid than H2SeO4
ii) Nitrite (NO2-) is a stronger base than sulphite (SO32-)
iii) Hydronium ion (H3O+) is a stronger acid than water (H2O)
(For the above, write acid dissociation reaction for each, and comment on the relative strengths of the conjugate bases formed in each case.)

Answer 1

iii. HA + H2O ==> H3O^+ + A^-
Weak acids placed in water solution provides H3O^.
The conjugate pair is H2O--H3O^+. Of that pair H3O^+ is the acid and H2O is the conjugate base.

Answer 2

HClO4 + H2O ==> H3O+ + ClO4^-
H2SeO4 + H2O ==> H3O^+ + HSeO4^- ==> We can compare only ka1 here. You know if the acid is stronger then the conjugate base is weaker.