Phosphoric acid H3PO4 is a triprotic acid meaning one molecule of the acid has three acidic protons estimate the pH and the concentration of all species in a 0.100M phosphoric solution.

Question

Ka1: 6.9x10^-3

Ka2: 6.2x10^-8

Ka3: 4.8x10^-13

find H3PO4 , OH- and pH

Answer 1

Typically, if the 2nd Ka value is 1000 times or more less than the 1st Ka value, it and subsequent dissociations can be ignored in determining pH and concentration of species. In the case of H₃PO₄, the 2nd Ka is 100000 less, so it and the 3rd dissociation can be ignored.

Looking at the dissociation of H₃PO₄ we have...

H₃PO₄ <==> H⁺ + H₂PO₄^2- Ka = 6.9x10^-3

6.9x10^-3 = [H⁺][H₂PO₄^2-] / [H₃PO₄] = (x)(x) / 0.1 - x

6.9x10^-3 = x² / 0.1-x

x² + 6.9x10^-3x - 6.9x10^-4 = 0

x = 0.0230 M = [H⁺] = [H₂PO₄^2-]

pH = -log [H^+]

pH = 1.64

[OH-] = 1x10^-14 / 0.023

[OH-] = 4.35x1^0-13 M

[H₃PO₄] = 0.100 M - 0.023 M

[H₃PO₄] = 0.0770 M