How should I apply the Henderson-Hasselbalch to a triprotic acid?
When Ka values are distinct enough (at least 3 pKa units apart) you can ignore other protons. Just use pKa2 (but don't forget about them when dealing with the neutralization stoichiometry!).
I've never seen the Henderson Hasselbalch applied to problems like these. This will be covered in most undergrad analytical chemistry books I buy, right? I will need to study it in more detail there.
With regards to this problem, let's say I can calculate the final analytical concentration in solution of NaOH and H3PO4. Can I write
C[H3PO4]=n[H3PO4]/V
total=(C
1[H3PO4]*V
1[H3PO4])/V
totalAnd then solve for V
total (n[H3PO4] can be worked out from the quantities given, as 0.05101 mol, and this does not change ever in this problem; C
1[H3PO4]=n[H3PO4]/V
1[H3PO4]=0.05101/0.250 if you really want but it isn't necessary) so V
total=n[H3PO4]/C[H3PO4] (C[H3PO4] is analytical conc in new solution) =(C
1[H3PO4]*V
1[H3PO4])/C[H3PO4] and this value of V
total can be used, with the analytical concentration of NaOH in the final solution (C[NaOH]) known, to calculate the original volume of NaOH used:
C[NaOH]=n[NaOH]/V
total=(C
1[NaOH]*V
1[NaOH])/V
totalAnd solve for V
1[NaOH]: V
1[NaOH]=C[NaOH]*V
total/C
1[NaOH], V
total worked out as before, C[NaOH] known, and C
1[NaOH] given as 0.80 M, so you get the volume of NaOH required.
Is this correct? Sorry for the maths/strange notation :p