Question

phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of acid has 3 acidic protons. estimate the pH and concentration of all species in a .200 M phosphoric acid solution Ka1=6.9x10^-3, Ka2=602x10^-8, Ka3=4.8x10^-13

Answer 1

initial [H3PO4] = 0.200 M

H3PO4 + H2O <==> H2PO4- + H3O+ --- Ka1 = [H2PO4-][H3O+]/[H3PO4] = 6.9 x 10^-3

let x amount dissodicated

6.9 x 10^-3 = x^2/0.200

x = 0.037 M

[H3PO4] = 0.200 - 0.037 = 0.163 M

[H3O+] = 0.037 M

pH = -log[H3O+] = 1.43

[OH-] = 1 x 10^-14/0.04 = 2.7 x 10^-13 M

H2PO4- + H2O <==> HPO4^2- + H3O+ --- Ka2 = [HPO4^2-][H3O+]/[H2PO4-] = 6.2 x 10^-8

let x amount dissociated

6.2 x 10^-8 = x^2/0.037

x = 4.8 x 10^-5 M

[H2PO4-] = 0.037 - 4.8 x 10^-5 = 0.037 M

HPO4^2- + H2O <==> PO4^3- + H3O+ --- Ka3 = [PO4^3-][H3O+]/[HPO4^2-] = 4.8 x 10^-13

[PO4^3-] = 4.8 x 10^-13 M

Overall,

[H3PO4] = 0.200 - 0.040 = 0.163 M

[H3O+] = 0.037 M

pH = -log[H3O+] = 1.43

[OH-] = 2.7 x 10^-13 M

[H2PO4-] = 0.037 M

[HPO4^2-] = 4.8 x 10^-5 M

[PO4^3-] = 4.8 x 10^-13 M