# Homework Help: Determing Ka values of H3PO4

1. Nov 30, 2012

1. The problem statement, all variables and given/known data
I need to determine the Ka values of H3PO4 having titrated a solution of H3PO4 and HCl with 0.1142 M NaOH. The concentrations of H3PO4 and HCl were found to be 0.106 M and 0.003 M respectively. The first EP was reached at 4.76 mL with a pH of 4.65 and the second EP was reached at 9.41 mL and a pH of 9.23.

2. Relevant equations

Kw=Ka*Kb
pH=pKa+log([A-]/[HA])
Ka=([H+][H2PO4-])/[H3PO4]

3. The attempt at a solution
Struggled with this for a while now and I feel like I am missing something obvious.

Attempted to use the fact that pKa = pH at 1/2 the EP for the second EP received an answer of 1.51*10^(-7), which is about an order of magnitude off of the accepted pKa2 of 6.8*10^(-8). Not sure how this could be - all of the HCl has been neutralized and you are just converting H2PO4- to HPO4 2-, however this is all based off data I collected. I'm not even sure how to determine the first Ka value or even where to start. I don't believe you can say that pKa=pH at 1/2 the first EP because you still have the HCl in solution. Not sure how else you could go about solving it. Any help would be great appreciated.

2. Dec 1, 2012

### Staff: Mentor

Have you taken into account thermodynamic corrections?