Question

Phosphate, with a pK2 of 6.82, will be mostly in the HPO4(2-) form at pH 7.2. At pH 5.82 it is mostly in the _________________________ forms.

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

Answer 1

a) H2PO4(-) and HPO4(2-)

At pH 5.82, phosphate is primarily in the H2PO4- and HPO4^2- forms, with the former being more prevalent due to the proximity of the pH to the pK2 value of 6.82.

Step-by-step explanation:

At pH 7.2, phosphate is mostly in the HPO42- form due to its pK2 value of 6.82. When the pH is lowered to 5.82, it shifts the equilibrium causing the phosphate to be mostly in the form of H2PO4- (dihydrogen phosphate) and to a lesser extent HPO42- (hydrogen phosphate), since the pH is close to the pK2, favoring the protonated form. Therefore, the correct answer is a) H2PO4- and HPO42- forms.