Question

The pKa of the amine group of the amino acid serine is 9.15. A- What is the pH of a solution where the ratio of the acidic form of serine to the basic form of serine is 60.0? B-What is the percent dissociation of serine at the pH in part A?

Answer 1

A)

pKa of serine = 9.15

pH = pKa + log [base / acid]

      = 9.15 + log [1 / 60]

      = 7.37

pH = 7.37

B)

Ka of serine = 10^-pKa = 7.08 x 10^-10

[H+] = 10^-pH = 10^-7.37

        = 4.27 x 10^-8 M

[H+] = sqrt (Ka x C)

4.27 x 10^-8 = sqrt(7.08 x 10^-10 x C)

C = 2.57 x 10^-6

concentration of serine = 2.57 x 10^-6 M

percent dissociation of serine = ( [H+] / C ) x 100

                                               = (4.27 x 10^-8 / 2.57 x 10^-6 ) x 100

percent dissociation of serine = 1.66%