Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of th

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 0.300 M phosphoric acid solution. PLEASE ANSWER ALL PARTS. NOT FAIR that I paid and I get half an answer.

Phosphoric acid, H3PO4aq), 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 0.300 M phosphoric acid solution, 2.16 7.21 12.32 Number Number Number Number Number HPO Number pH = Number PO

Explanation / Answer

pKa = -logKa

Ka1 = 6.92 x 10^-3 ; Ka2 = 6.16 x 10^-8 ; Ka3 = 4.79 x 10^-13

H3PO4 <==> H+ + H2PO4- ------ Ka1 = [H2PO4-][H+]/[H3PO4]

let x amount has dissociated

6.92 x 10^-3 = x^2/0.3

x = [H+] = 0.0455 M

pH = -log[H+] = 1.34

[OH-] = 1 x 10^-14/0.0455 = 2.20 x 10^-13 M

[H3PO4] = 0.3 - 0.0455 = 0.2545 M

H2PO4- <==> H+ + HPO4^2- ------- Ka2 = [HPO4^2-][H+]/[H2PO4-]

let x amount has dissociated

6.16 x 10^-8 = x^2/0.0455

x = [HPO4^2-] = 5.30 x 10^-5 M

[H2PO4-] = 0.0455 - 5.30 x 10^-5 = 0.0454 M

HPO4^2- <==> PO4^3- + H+ ------- Ka3 = [PO4^3-][H+]/[HPO4^2-]

since Ka3 is very small, dissociation at this stage is also small.

hence, [PO4^3-] = 4.79 x 10^-13 M

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