what is the pka of anhydride? i've searched the whole web and can't seem to find the pka of the functional group.

I don't think you will be able to find the Ka or pKa of a group. If you name the anhydride you can find the pKa but it will vary with what's attached to the functional group. For example, HCOOH has a different pKa than CH3COOH so the "so called" pKa for the COOH group depends upon the molecule and not upon the group itself.