Question

Malic acid is a weak diprotic organic acid with K1 = 4.0x 10-4 and K2 = 9.0 x 10-6.
(a) Letting the symbol H2A represent malic acid, write the chemical equations that
represent K1 and K2 . Write the chemical equation that represents K] xK2 .
(b) Qualitatively describe the relative concentrations of H2A, HA-, A 2-, and H30+ in a
solution that is about one molar in malic acid.
(c) Calculate the pH of a 0.0100 M malic acid solution and the equilibrium
concentration of [H2A].
(d) What is the A 2- concentration?

Answer 1

I would like to know what you don't understand about this. It would help answer the questions.
H2A ==> H^+ + HA^-
HA^- ==> H^+ + A^2-

Surely you know how to write Ka1 and Ka2 expressions. If not explain and I can help you through them.

b. (H2A) > (H^+) = (HA^-) > (A^2-)

c. .........H2A ==> H^+ + HA^-
I..........0.01.....0......0
C...........-x......x......x
E........0.01-x....x......x

Substitute the E line into Ka1 expression and solve for x = H^+, then convert to pH. (H2A) = 0.01-x

d. (A^2-) = ka2. Why? Write Ka2 expression.
ka2 = (H^+)(A^2-)/(HA^-)
From Ka1 (see c above), (H^+) = (HA^-) so (H^+) and (HA^-) cancel each other in Ka2 expression, so (A^2-) = Ka2.