Question

Phosphate, with a pK1 of 2.15 and a pK2 of 6.82, will be mostly in the HPO₄(2-) form at pH 7.2. At pH 1 it is mostly in the _________________________ forms.

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

Answer 1

At pH 1, phosphate is mostly in its protonated forms which are H3PO4 (phosphoric acid) and H2PO4- (dihydrogen phosphate ion), because the low pH suppresses the dissociation of protons.

Step-by-step explanation:

At pH 1, which is highly acidic, phosphate will be mostly in the forms that have the most protons because at lower pH, the tendency to donate protons is low, and hence substances are more likely to be in their protonated forms. In the case of phosphate, the pK1 is 2.15 for the first proton dissociation (conversion from H3PO4 to H2PO4-) and the pK2 is 6.82 for the second dissociation (conversion from H2PO4- to HPO42-). Since at pH 1, the pH is well below both pK1 and pK2, the predominant species will be those that are more protonated.

Therefore, at pH 1, phosphate is mostly in the H3PO4 (phosphoric acid) and H2PO4- (dihydrogen phosphate ion) forms. This reflects the fact that in an acidic environment, dissociation of protons from phosphate is suppressed.