Question

4. A diprotic acid has the following pK values: pKa1 = 5.50, pKa2 = 9.50

A. At what pH does [H2A] = [HA- ]?

B. At what pH doe [HA- ] = [A2- ]?

C. Which is the principal species at pH = 3.50, H2A, HA- , or A2- ? Justify your answer using the Henderson-Hasselbach equation.

D. Which is the principal species at pH = 7.5, H2A, HA- , or A2- ? Justify your answer (don’t use Henderson-Hasselbach equation, this is a half-neutralized diprotic acid).

E. Which is the principal species at pH = 9.8, H2A, HA- , or A2-? Justify your answer using the Henderson-Hasselbach equation

Answer 1

a)

for

pH = pKa + log(HA-/H2A)

pH = pKa1 + log(1)

pH = 5.5

b)

same case,

pH = pKa2 = 9.50

c)

at pH 3.5

expect H2A, HA- the most ... especially H2A

since pH range is in pKa1

d)

pH = 7.5 ; expect similar amounts of Ha- and A2- .. the most material is HA-. Still H2A is the most

e)

wehn pH = 9.5

the most species are HA- and A-2