Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that
one molecule of the acid has three acidic protons. Estimate the pH,
and the concentrations of all species in a 0.350 M phosphoric acid
-log(Ka1)= 2.16, Ka1= 10(-2.16) =0.006918
The dissolution can be written as
H3PO4----> H2PO4- + H+
Ka1= [H2PO4-] [H+]/ [H3PO4]
let x= degress of disociation of H3PO4
at Equilibrium [H3PO4]= 0.35-x [H+] = [H2PO4-]
The quadratic equation can be solved by either tiral and error method or solving the quadratic equation
[H+] =0.04586= [H2PO4-]
for the second case ,i.e H2PO4- ---> HPO4- + H+
Ka2= [HPO4-][H+]/ [H2PO4-] = x2/(0.04586-x)
pKa2= 7.21 Ka2= 6.17*10-8
the order of magnitude of Ka2 , ka3 and concentration of H2PO4- suggest the concentrations of H+ will be less.
hence [H+] can be neglected for second and third cases.
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