Calculating Formic Acid & Sodium Formate for pH Change

[zeshkani]

Homework Statement

A solution is to be buffer at pH = 3:35 with formic acid and formate.
(a) How many moles of formic acid and sodium formate are needed if the solution is to change pH by < 0:02
for the addition of 0.02 moles of strong acid?

i need help how to set up a problem like this, i don't even know how to approche it !

 

[Stevedye56]

I think this might help. Henderson-Hasselbalch