Calculate the pH and include the balanced equation for the acid dissociation reaction.
0.20 M HC3H5O2 (propionic acid, Ka = 1.3 x 10^-5)
To save on typing let's call propionic acid HP.
........HP ==> H^+ + P^-
I......0.20....0.....0
C.......-x.....x.....x
E.....0.20-x...x.....x
Substitute the E line into the Keq expression and solve for x = (H^+), then convert to pH.