Question

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 solution.
​pKa1=2.16
pKa2=7.21
pKa3=12.32

Answer 1

PKa1=2.16

-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-]

x²/(0.35-x)= 0.006918

The quadratic equation can be solved by either tiral and error method or solving the quadratic equation

x= 0.04586

[H+] =0.04586= [H2PO4-]

pH= -log(0.04586)=1.338

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.