The equilibrium chemical reaction is
              Â
HCOOH(aq) + H2O(l) <------------->Â Â HCOO-(aq)
  +   H3O+(aq)
IÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0.30
MÂ 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 Â Â Â Â Â Â Â Â
0
CÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
-x                             Â
   Â
+x                     Â
+x
EÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0.30-x                               Â
 Â
+x                    Â
+x
Ka = [HCOO-][H+] / [HCOOH]
1.70 x 10^-4 = x^2 / (0.30-x)
x^2 + (1.7 x 10^-4)x -(0.51 x 10^-4) = 0
Solving the quadratic equation, we get the roots
x = -0.007
x = +0.007
Thus, we use x = +0.007
Thus, the hydronium ion concentration = [H3O+] = x =
0.007 M
