Question

A weak acid HA has a pKa of 3.75. In a solution with pH = 4.12, what fraction of the weak acid is in the A-(aq) form?

(a) 1.8 x 10-4 (b) 0.30 (c) 0.43 (d) 0.70

Answer 1

answer : (c) 0.43

solution:

pH = 1/2 [pKa - logC]

4.12 = 1/2 [3.75 - log C]

-log C = 4.49

C = 3.23 x 10^-5 M

[H+] = 10^-pH

         = 10^-4.12

          = 7.59 x 10^-5 M

[H+]    = A- = 7.59 x 10^-5 M

fraction of A-   = C / A

                       = 3.23 x 10^-5 / 7.59 x 10^-5  

                      = 0.43