Phosphoric acid H3PO4(aq), is a triprotic acid, meaning that one molecule of the
ID: 820826 • Letter: P
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 0.200 M phosphoric acid solution.
pKa1=2.16
pKa2=7.21
pKa3=12.32
[H3PO4]=
[H2PO4^-]=
[HPO4^2-]=
[PO4^3-]=
[H+]=
[OH-]=
pH=
Explanation / Answer
ka1 = 6.91 * 10^-3
ka2 = 6.16 * 10^-8
ka3 = 4.78 * 10^-13
You can get these by formula: pka = - log (ka)
write 3 equilibria of dissociation of H3PO4
H3PO4 ? [H+] + [H2PO4-]
[H2PO4-] ? [H+] + [HPO4^2-]
[HPO4^3-] ? [H+] + [PO4^3-]
Where ka1, ka2 and ka3 are the respective equilibrium constants.
In the first equilibrium, we cannot ignore the decreasing concentration of [H3PO4] and we have a quadratic for the value of concentration of [H+] and thus concentration of [H+] is 4.4 * 10^-2.
This implies that pH is 1.3566. [H2PO4-] is same as [H+] and [H3PO4] = 0.2 - 4.4 * 10^-2 = 0.156. [HPO4^2-] = ka2 = 6.16 * 10^-8. [PO4^3-] = 6.69 * 10^-19. All concentrations are expressed in M. Concentration of [OH-] = 2.27 * 10^-13.
Note that the addition of [H+] due to the second and third equilibrium is very low, and so we can neglect it.
These are simultaneous equilibria