What is the ph of a 2.2 m solution of hclo4?

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