Equilibrium constants for the ionization of phosphoric acid at 25C and 1 bar are
ID: 1087104 • Letter: E
Question
Equilibrium constants for the ionization of phosphoric acid at 25C and 1 bar are
H3PO4 (aq) = H+ + H2PO4- K1 = 10^-2.1
H2PO4- = H+ + HPO4^2- K2 = 10^-7.2
HPO4^2- = H+ + PO4^3- K3 = 10^-12.2
From these activity products, would you estimate that phosphoric acid is stronger or weaker than carbonic acid? What would be the principal phosphate ion or ions in a solution at pH=4? at pH=10? If calcium phosphate, Ca3(PO4)2 (K = 10^-28.7), were place in contact with each of these solutions, how much would dissolve?
Explanation / Answer
Phosphoric acid will be stronger acid than carbonic acid because the first ionisation constant of phosphoric acid is higher than ionisation constant of carbonic acid.
At pH approx.4,there will be 0.01 moles of Ca (OH)2 and 0.02 moles of H3PO4.At PH approx. 10,HPO42- and PO43- will be present.Conc. of Ca (OH)2 and H3PO4 will be same as total conc. of Ca and phosphate respectively
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