[minimal]

Hi, i'm doing review for a test, and the question is:

For a triprotic acid, H3A,
Ka1 is 1.0 x 10^-2
Ka2 is 1.0 x 10^-6
Ka3 is 1.0 x 10^-10

The pH range in which H2A- is the predominant form is a pH between
(a) 1 and 3
(b) 3 and 5
(c) 5 and 7
(d) 7 and 9
(e) 9 and 11
?

Now, I know the answer is B, because I looked, but I would like some help in figuring out why.  My guess is that H2A- will be the dominant form at anything between 2 and 6.  Because the pKa of the most acidic proton is 2, and the second most acidic proton is 6.  I know when the pH is equal to the pKa, there is 50% acid/conj base present.  So anything above 2 and below 6 should be mostly H2A-, right?  Why did they choose 3 and 5?

[macman104]

Exactly, above 2 and below 6.  The question is being very specific with its words.  Because it will be 50/50 at the pKas, then H2A^- technically doesn't predominate at 2 and 6.