Answer to Question #8271 in Inorganic Chemistry for will

Question

Is H3PO4 a triprotic or diprotic acid, if it is a trprotic acid, then how do we explain its reaction with sodium hydroxide? i.e. H3PO4 + 2NaOH == Na2HPO4 + 2H2O.

Answer 1

H₃PO₄ is a trprotic acid,but the reactivity of cleavage ща proton determine the dissociation constant. For the third proton is much smaller than for the second and first
H₃PO_4(s) + H₂O_(l) H₃O⁺_(aq) + H₂PO₄^−_(aq)
K_a1 = 7.25×10^−3
H₂PO₄^−_(aq) + H₂O_(l) H₃O⁺_(aq) + HPO₄^2−_(aq)
K_a2 = 6.31×10^−8
HPO₄^2−_(aq) + H₂O_(l) H₃O⁺_(aq) + PO₄^3−_(aq)
K_a3 = 3.98×10^−13