Question

Malic acid is a diprotic acid with K1=3.5E-4 and K2=8.0E-6

a) What is the pH of a .1M solution of malic acid? Check all approximations

b) How much dissociation occurs in part a)?

c) What are the dominant species at a pH of 5.5?

d) What is the pH of a buffer formed by mixing 10 ml of .1 M sodium hydrogen maleate (NaHM) and 10 ml of .1 M disodium maleate (Na2M)?

Answer 1

a)

H2A -------------------> H+ + HA-

0.1                            0           0          ----------------> initial

0.1-x                        x            x -----------------> equilibrium

Ka1 = (x)^2 / (01.-x)

3.5 x 10^-4 = x^2 / 0.1-x

x^2 + 3.5 x 10^-4 -3.5 x 10^-5 = 0

x = 5.74 x 10^-3

x = [H+] = 5.74 x 10^-3 M

pH = -log[H+]

pH = -log (5.74 x 10^-3 )

pH = 2.24

(b) dissociation % = x / 0.1 ) x 100

                           = 5.74 x 10^-3 / 0.1 ) x 100

                          =5.74 %

(c) at pH = 5.5 dominent species   C4H5O-   & C4H4O-2 ( hydrogenmaleate ion & maleate ion )

(d) K2=8.0E-6

     pK2 = -log (8.0E-6) = 5.097

pH = pK2 + log [Na2M/ NaHM]

pH = 5.097 + log (0.1 x 10/ 0.1 x 10)

pH = 5.097