To determine the pH of a solution which has 0.06 m in sodium benzoate (nac7h5o2) and 0.09 m in benzoic acid (hc7h5o2), we use the ICE table and the acid dissociation constant of hc7h5o2 to determine the concentration of the hydrogen ion present at equilibrium. We do as follows:
HC7H5O2 = H+ + C7H5O2-
NaC7H5O2 = Na+ + C7H5O2-
Therefore, the only source of hydrogen ion would be the acid. We use the ICE table,
HC7H5O2 H+ C7H5O2-
I 0.09 0 0.06
C -x +x +x
------------------------------------------------------------------
E 0.09-x x 0.06+ x
Ka = 6.28x10^(-5) = (0.06 + x) (x) / 0.09 -x
x = 9.40x10^-5 M = [H+]
pH = - log [H+]
pH = -log 9.40x10^-5 M
pH = 4.03
Therefore, the pH of the resulting solution would be 4.03.