Answer:
pH=4.05
Explanation:
C7H6O2 -> C7H6O- + H+
ka= [C7H6O-] [H+]/[C7H6O2]
During equilibrium
[C7H6O-]= [H+]= x^2
[C7H6O2]=0.12-x
Replace
ka= x^2/0.12-x
6.5 x10^-5= x^2/0.12-x
7.8x10^-6 - 6.5 x10^-5x=x^2
x^2+ 6.5 x10^-5x - 7.8x10^-6
Solution of quadratic equation
x=8.8 x10^-5
pH= -log [H+]= -log 8.8 x10^-5=4.05
