Question

A chemist prepares a solution that contains a formal concentration of 0.1 phthalic acid and that has a pH of 8.12

Phthalic acid:

Pka1: (HOOC6H4COOH/HOOCC6H4COO-) = 2.95

Pka2: (HOOCC6H4COO-/-OOCC6H4COO-) = 5.41

a) calculate the fraction ofeach species: HOOCC6H4COOH, HOOCC6H4COO-, and -OOCC6H4COO- at this pH and decide which one is the predominant form.

b) plot an appromimate curve of alpha vs PH for each species

Answer 1

For phthalic acid

pH = -log[H+] = 8.12

[H+] = 7.6 x 10^-9 M

Ka1 = 0.0011 ; Ka2 = 4 x 10^- 6

[A] = [H+]^2 + Ka1[H+] + Ka1Ka2

      = (7.6 x 10^-9)^2 + 0.0011 x 7.6 x 10^-9 + 0.0011 x 4 x 10^-6

      = 5.78 x 10^-17

alpha[HOOCC6H4COOH] = [H+]^2/[A] = 7.77 x 10^19

alpha[HOOCC6H4COO-] = Ka1[H+]/[A] = 1.44 x 10^5

alpha[-OOCC6H4COO-] = Ka1Ka2/[A] = 7.61 x 10^7

So the predominant species at pH 8.12 would be, HOOCC6H4COOH

The undissociated phthalic acid.