Question

I posted a question several days ago: The pH of human blood needs to be between 7.35 and 7.45. You want to prepare a buffer solution that gives a pH of 7.40. You decide to use a sodium phosphate buffer: the acid is
H2PO−4 and the conjugate base is HPO2−4
. You want the concentration of the acid to be 0.0100 M.
1. If the initial H2PO−4 concentration is 0.0100 M, what is the initial concentration of
HPO2−4 that will give a pH of 7.40?
2. What is the maximum molarity of acid that this buffer can neutralize without the pH dropping below 7.35?
3. What is the maximum molarity of base that this buffer can neutralize without the pH
going above 7.45?

I solved the first question of it, but am lost on the second and third question.

Answer 1

Even though it isn't quite correct since we don't write subscripts on this forum, you should use HPO4- and H2PO42- (I prefer HPO4^- and H2PO4^2-). Most will not know what HPO2-4 is. I responded to this problem a few days ago.
Frankly, I don't think 2 and 3 can be answered with the information given. You must know the M, of course, BUT you also must have the volume of acid (for 2) and base (for 3) in order to know how many mols are being added. There must be some other information that you've not included.