Question

Phosphate, with a pK2 of 6.82, will be mostly in the H₂PO₄(1-) form at pH 5.82. At pH 7.2 it is mostly in the _________________________ forms.

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

Answer 1

At a pH of 7.2, phosphate is primarily in the hydrogen phosphate (HPO4^2-) and phosphate (PO4^3-) forms, corresponding to answer choice a) HPO4^2- and H2PO4^1-.

Step-by-step explanation:

At pH 7.2 phosphate will be mostly in the HPO42- (hydrogen phosphate) and PO43- (phosphate) forms. This is because as the pH rises above the pK2 value of phosphate (6.82), deprotonation occurs leading to the formation of these two ionic species. Specifically, under physiological conditions at pH ~7.4, phosphoric acids lose protons and exist predominantly as these phosphate anions. Therefore, the correct answer is a) HPO42- and H2PO41- forms.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.