Question

A phosphate buffer solution at a pH = pK3 = 12.38 would have equal amounts of phosphate in the ____________ form and the HPO₄(2-) form.

a) H₃PO₄ and H₂PO₄(1-)
b) HPO₄(2-) and H₂PO₄(1-)
c) H₂PO₄(1-) and PO₄(3-)
d) HPO₄(2-) and PO₄(3-)

Answer 1

At a pH equal to pK3, a phosphate buffer would have equal concentrations of the hydroxide phosphate (HPO₄²-) and the phosphate (PO₄³-) forms, which corresponds to answer option (d).

Step-by-step explanation:

A phosphate buffer solution at a pH = pK3 = 12.38 would have equal amounts of phosphate in the HPO₄²- form and the PO₄³- form. Therefore, the correct answer is (d) HPO₄²- and PO₄³-.

At the pK3 of phosphoric acid, the third dissociation step is the relevant equilibrium, signified by the reaction HPO₄²- ↔ H⁺O⁺⁴⁺ + PO₄³-. Because pH equals pK3, it implies that the concentrations of HPO₄²- (hydrogen phosphate) and PO₄³- (phosphate) are equal. This equality of concentrations at pH equals pK3 is a characteristic trait of buffer systems during their respective pKa (acid dissociation constant) value.