Calculate the pH of the solution prepared when 0.2 mol HClO4 is added to 2.00 L of a solution that is 0.500 M CH3CO2Na. (Ka of CH3CO2H = 1.8e-5.

HCLO4 here is the strong acid
so it is going to react with CH3CO2- 
HClO4 + CH3CO2-  ----> CH3COOH + ClO4-
I o.2 mol   ,1.0mol            0                  0
after  0          1.0-0.2          0.8               0.8
CH3COOH +H2O  <----->CH3COO-   + H3O+
now make ice table or use Henderson equation as we discussed in the session to take out the pH

Q75 of the HW packet (requested by you )
Calculate the mass of (NH4)2SO4 that must be added to 0.50 M NH3 in order to prepare a buffer
with a pH of 8.65. (Ka of NH4 + = 5.6 x 10–10)
use henderson hasselbalch equation
pH = pKa +log[(NH4)2SO4]/[NH3]
you have pH ,pKa and [NH3]
plug in all values to take out the [(NH4)2SO4]
From this calculate moles in one Litre of the solution and multiply it by molar mass to get the mass of the (NH4)2SO4 required for making that buffer.