find pH of 0.050 M HCLO4 plus 0.050 M HBr> You must consider activities>

Answer 1

I think you need the volumes of each although you can assume equal volumes, I think, since both are strong acids.
Are you to look up the activities or calculate them from the Debye-Huckel equation or the extended D-H equation. In any event, you will need to calculate the ionic strength, then calculate activity using DH or look up in a table. PH comes from that, of course, as pH = -log(H^+).