Finding pH of 0.40M NH4ClO4 without given Ka

[I'm]

Homework Statement

I was asked to calculate the pH of .40M solution of NH4ClO4

but I wasn't given a Ka for it.

Homework Equations

The Attempt at a Solution

I dissciated NH4ClO4 into:

NH4(plus) and ClO4(minus)

but I"m stuck there. What am I doing wrong?

 

[Borek]

So far OK. Determine which ion is responsible for pH change. There are pKa (Ka) tables - use them.