Question

Determine the pH of each of the following solutions.

(a) 0.821 M hydrogen peroxide (weak acid with K_a = 2.4e-12).





(b) 0.306 M boric acid (weak acid with K_a = 5.8e-10).





(c) 0.309 M pyridine (weak base with K_b = 1.7e-09).

Answer 1

pH calculation

(a) 0.821 M H2O2

let x amount has dissociated

Ka = [HO2-][H+]/[H2O2]

2.4 x 10^-12 = x^2/0.821

x = [H+] = 1.403 x 10^-6 M

pH = -log[H+] = 5.853

(b) 0.306 M H3BO3

let x amount has dissociated

Ka = [H2BO3-][H+]/[H3BO3]

5.8 x 10^-10 = x^2/0.306

x = [H+] = 1.332 x 10^-5 M

pH = -log[H+] = 4.875

(c) 0.309 M pyridine

let x amount has been protonated

Kb = [Hpyridine+][OH-]/[pyridine]

1.7 x 10^-9 = x^2/0.309

x = [OH-] = 2.292 x 10^-5 M

pOH = -log[OH-] = 4.64

pH = 14 - pOH = 9.36