Question

C. Calculate the pH of a 0.10 M aqueous solution of the weak base ammonia, NH3 , if the degree of dissociation is 1.34%.

Answer 1

                                        NH₃    +   H₂O       NH₄⁺ + OH^-

Initial concentration          C                                0             0

Change in concentration   -C                          C          C    (where = degree of dissociation)

Equilibrium concentration C(1-) C          C

Therefore the concentration of OH^-, [OH-] = C

                                                                    = (0.10 x 1.34)/100

                                                                    = 0.00134 M

Now, [H+] = K_w/[OH^-]

                   = 10^-14/0.00134

                   = 7.46 x 10^-12

or, - log[H+] = - log[7.46 x 10^-12]

or, pH = 11.13

Therefore, pH of the aqueous solution = 11.13