Question

The acid H₂A has pK₁ = 4.00 and pK₂ = 8.00.

(a) At what pH is [H₂A] = [HA^-]?

(b) At what pH is [HA^-] = [A^2- ]?

(c) Which is the principal species, H₂A, HA^-, or A^2- at pH 2.00?

(d) Which is the principal species at pH 6.00?

(e) Which is the principal species at pH 10.00?

Please answer this thoroughly so I can understand your setup and approach. Thank you!

Answer 1

H2A <----> HA^- + H^+

ka1 = [HA-][H+]/[H2A]

HA^- <-----> A^2- + H^+

KA3 = [A^2-][H+]/[HA-]

if acidic bufefr pH range usually from (pka1-1) to (pka1+1)

pH = pka1 + log(HA-/H2A)

a) If HA^- = H2A

pH = pka1 = 4

if acidic bufefr pH range usually from (pka2-1) to (pka2+1)

b) pH = pka2 + log(A-2/HA-)

if A-2 = HA-

pH = pka2 = 8

c) pH = 2 < pka1 = 4. so that,

from pH = pka1 + log(HA-/H2A) formula

H2A must be principle species.

d) pH = 6 < pka2 = 8

from pH = pka2 + log(A-2/HA-) formula

    HA^- ions must be the principle species

e) pH = 10 > pka2 = 8

   A^2- ions is the major principle species.