H₃PO₄ has three acidic protons with the following Ka values:
Ka₁ = 7.1 x 10⁻³
Ka₂ = 6.3 x 10⁻⁸
Каз = 4.5 x 10⁻¹³
If you have this acid in a solution with a pH=5 what is the predominate form of the compound in solution?
PO₄³⁻