proponoic acid is a week acid
construct the ICE table
         C2H5CO2H
+ H2O <------> C2H5CO2- + H3O+
IÂ Â Â Â Â Â Â Â Â Â Â
0.02Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0
CÂ Â Â Â Â Â Â Â Â Â
-x                                             Â
+x             Â
+x
EÂ Â Â Â Â Â Â Â Â Â Â
0.02-x                                  Â
+x                 Â
+x
now dissociation constant
Ka = [C2H5CO2-][H3O+] / [C2H5CO2H]
3.4 x 10-5 = [x][x] / [0.02-x]
x2 + x 3.4 * 10-5 - 6.8 x 10-7 = 0
solve the quadratic equation to get x value
x = 0.0008078 M = [H3O+]
now
pH = -log[H3O+] = -log[0.0008078] = 3.09
