Question

Determine the pH of each of the following solutions.

(a) 0.123 M propionic acid (weak acid with Ka = 1.3e-05).

(b) 0.378 M hypoiodous acid (weak acid with Ka = 2.3e-11).

(c) 0.688 M pyridine (weak base with Kb = 1.7e-09).

Answer 1

pH determination,

(a) 0.123 M propionic acid

acid dissociates as, assuming HA represents as acid then,

HA <==> H+ + A-

let x amount has dissociated then,

Ka = [H+][A-]/[HA]

1.3 x 10^-5 = x^2/0.123

x = [H+] = 0.0103 M

pH = -log[H+] = 1.99

(b) 0.378 M HIO

again repeating the same process as in (a)

Ka = x^2/0.378

2.3 x 10^-11 = x^2/0.378

x = [H+] = 2.95 x 10^-6 M

pH = 5.53

(c) 0.688 M pyridine

base hydrolyzes in water as,

B + H2O <==> BH+ + OH-

let x amount has reacted then,

Kb = [HB+][OH-]/[B]

1.7 x 10^-9 = x^2/0.688

x = [OH-] = 3.42 x 10^-5 M

pOH = 4.46

pH = 9.53