Question

Calculate the molar concentrations of all phosphate species in H3PO4 in a solution with
pH = 2.25, given that the concentration of all four forms of phosphate total 15 mM.

Answer 1

Convert pH to (H^+).
Work this by the fractional method.
alpha o = (H^+)^3/D
alpha 1 = (H^+)^2*k1/D
alpha 2 = (H^+)^k1k2/D
alpha 3 = k1k2k3/D

D is
(H^+)^3 + (H^+)^2*k1 + (H^+)*k1k2 + k1k2k3

Then (H3PO4) = alphao*15 mM
(H2PO4^-) = alpha1*15 mM
etc.