Hi again Babcock_Hall,
OK, here's my stab at it:
1) First things first, converting Molar to moles for each compound involved.
1a) 0.1L*0.2mol/L => 0.02mol unknown compound
1b) 0.04L*0.2mol/L => 0.0008mol HCl
2) Find original mol of base::acid
Therefore, there are 0.004mol of the base and 0.016mol of the acid ORIGINALLY.
3) Find change in mol of base::acid
Change is from 1b) above, i.e. 0.0008 mol acid added, 0.0008 mol base subtracted. The final amounts of base and acid are 0.0032 mol and 0.0168 mol, respectively.
4) I just thought of this now... maybe you can confirm if I'm correct? I think I need to plug in the new base::acid amounts and the new pH, then calculate the new pKa...right? (maybe?)
If 4) above is correct, could you explain why? I just guessed. I don't understand the reasoning behind it, it just seemed to work with the numbers I had up until that point. Thanks!