[Bacter]

Hullo, and preemptively thank you for any assistance

So the question is:

4)    What is the pH of each of the following buffer mixtures?  (Phosphate pKa’s = 2.14, 6.86, 12.4)
A)      0.7 M acetic acid plus 0.1 M sodium acetate.
B)      0.4 M H3PO4 plus 0.8 M KH2PO4

Now since this is the pH of a buffer mix, I'm immediately thinking Henderson-Hasslebalch. This seemed pretty straightforward with part A

pH = pKa + log([A-]/[HA])
pKa for acetic acid = 4.76
Since the pKa is over 2.5 I assume that it's solely responsible for concentrations of acid and conjugate base
therefore [A-] = 0.1, [HA] = 0.6, and pH = 4.76 + log(0.1/0.6) = 3.98

Please note if that's totally off lemme know please 

BUT THEN we get to part B

Why would I get three pKas?
Are there multiple removals of hydrogens? Am I being given TMI as a red herring? Any help would be appreciated!

[Borek]

[HA] = 0.6

You were OK till now.

Why would I get three pKas?

Cause H₃PO₄ is a triprotic acid?

[Bacter]

Ok so... did I get the [HA] and [A-] reversed?

Also yes, I see the triprotic acid thing. How's that gonna impact the determination of the buffer pH?

[Borek]

Ok so... did I get the [HA] and [A-] reversed?

No. HA is given in the question.

Also yes, I see the triprotic acid thing. How's that gonna impact the determination of the buffer pH?

It won't assuming pKa is not too low nor too high and is far enough from other pKas.