Since HClO4 is a strong acid it will completely dissociate in solution.
HClO4 + H2O --> H3O+ + ClO4-
Therefore, your concentration of HClO4 will be equal to your amount of H3O+, the conjugate acid in solution.
The Ka expression is as follows:
Ka= [H3O+][ClO4-] / [HClO4]
So you know that -log [H3O+] gets you pH, so just plug in [2.2] as your concentration of H3O+ and your answer should be -.34 pH
