Squeezing in one more question...

I have a question about the Winter 2006 Exam (3). I'm looking at #2, asking whether a combination of compounds result in effective buffer solutions.One combination is 1.28mol Na2CO3(aq) + 0.64mol HCl (aq). I think this does make an effective buffer solution because the strong acid converts 0.64mol of Na2CO3 (a weak base) into its conjugte acid. Therefore you'd have equal amounts of a weak base and its conjugate acid. Is this correct reasoning so far?However, another combination is 0.90mol Na3PO4(aq) + 1.35mol HNO3 (aq). Here there is more strong acid than there is weak base, so all of Na3PO4 would be converted into its conjugate acid...that wouldn't result in a good buffer, correct? The answer key says that this combination would make a good buffer. Why?

But would it stop at monohydrogen phosphate ions? After all of the phosphate is neutralized to monohydrogen phosphate (a conjugate acid/conjugate base pair), half of the monohydrogen phosphate is neutralized to dihydrogen phosphate. This is an equimolar mixture of a weak conjugate acid (dihydrogen phosphate) and its weak conjugate base (monohydrogen phosphate). For most polyprotic acids (or bases), we can make more than one effective buffer by using each of the protonation equilibria. For phosphate, there are three buffers possible at three different pH's: phosphate/monohydrogen phosphate at ~12, monohydrogen phosphate/dihydrogen phosphate at ~7, and dihydrogen phosphate/phosphoric acid at ~2.

Likewise, carbonate can make the buffer mentioned in the problem (carbonate/bicarbonate at ~10) as well as bicarbonate/carbonic acid buffer at ~6.