[daylight]

A solution is prepared that is 0.55M in phosphoric acid(Ka1=7.1x10^-3,Ka2=6.3x10^-8, Ka3=4.3x10-13). Find the [H3O][OH]ph andpOH.
So I attempted it, but something seems off. Here is my attempt. I used ICE
H3PO4 + H20 <>H3) + H2PO4   H2PO4 + H2O <> H3O + hpo4    HPO4 + H2O<> H3O + PO4

      H3PO4          H30             HCO
I       .55              0                0
C      -X              +X                +X
E      (.55-X)        x                   X

Ka1 = 7.1x10^-3= [H3O][HCO3]/H2PO4)=x^2/(.55-x)=     -x^2 + -7.1x10^-3 + 3.9x10^-3   
insert in the quadratic:
and I get this
7.1x10-3 +- .125/1.42x10^-2 = p.31 for + and -8.31 for-
use 9.31=[H30]
ph=-log[H3O]=-.97

and that is where I stopped, it just doesn't seem right for my pH to be a negative number???

[Borek]

pH can be negative, but that's not the case here. Your ICE table looks OK (apart from the fact that there are no HCO₃^- in the solution).

[daylight]

OPPS that's from the problem above it It should be H2PO4..

[AWK]

wrong solutium of your correct quadratic equation