Question

The AIDS drug zalcitabine (also known as ddC) is a weak base with the structure shown below and a pKb of 9.8.

What percentage of the base is protonated in an aqueous zalcitabine solution containing 565 mg/L ?   
(Answer to 2 significant figures)

Answer 1

pKb = 9.8

Kb = 10^-9.8 = 1.58 x 10^-10

concentration = 565 mg / L

                       = 0.565 g / L

                       = 0.565 / 211.218

                       = 2.68 x 10^-3 M

[OH-] = sqrt (Kb x C)

          = sqrt(1.58 x 10^-10 x 2.68 x 10^-3)

[OH-] = 6.50 x 10^-7 M

% protanated = [OH-] / C ) x 100

                    = (6.50 x 10^-7 / 2.68 x 10^-3 ) x 100

% protanated = 0.024 %