Question

A weak acid (HA) has a pKa of 4.544. If a solution of this acid has a pH of 4.923, what percentage of the acid is not ionized? (Assume all H in solution came from the ionization of HA.)

Answer 1

pH of weak acid = 1/2(pka-logC)

pka = 4.923 , c = Concentration of acid = ? M

4.544 = 1/2(4.923-logx)

C = concentration of acid = 6.84*10^-5 M

[H+] = 10^-pH

     = 10^-4.544

     = 2.86*10^-5 M

[H+] = cx

(2.86*10^-5) = (6.84*10^-5)*x

x = degree of dissociation = 0.418

percentage of dissociation = 0.418*100

                            = 41.8 %

percentage of un-dissociated acid = 100-41.8 = 58.2%