Question

Phosphate, with a pK1 of 2.15 and a pK2 of 6.82, will be mostly in the H₃PO₄ form at pH 0.5. At pH 4.5 it is mostly in the _________ forms.

a) H2PO₄⁻
b) HPO₄²-
c) PO₄³-
d) H₃PO₄

Answer 1

At pH 4.5, phosphate is primarily in the H2PO4- (dihydrogen phosphate) and HPO4^2- (hydrogen phosphate) forms, as pH 4.5 lies between the two pK values for phosphoric acid's dissociation. The option (A) is correct.

Step-by-step explanation:

At pH 4.5, phosphate is mostly in the forms of H2PO4− (dihydrogen phosphate ion) and HPO42− (hydrogen phosphate ion).

Considering the pK values given for phosphate, where pK1 for the conversion of H3PO4 to H2PO4− is 2.15 and pK2 for the conversion of H2PO4− to HPO42− is 6.82, pH 4.5 falls between these two values. Thus, at pH 4.5, phosphoric acid dissociates mostly into H2PO4−, but some of it further dissociates into HPO42−. Therefore, option (A) is correct.