The ion HPO4–2 has both a conjugate base as well as a conjugate acid. The conjugate base of this ion is (1) while the conjugate acid is (2).

Question

a) (1) = H2PO4– (2) H3PO4
b) (1) = PO4– (2) H2PO4+
c) (1) = PO4–3 (2) H2PO4–
d) (1) = HPO5–3 (2) HPO3–
e) (1) = H2PO4– (2) PO4–3

help! don't know what to do

Answer 1

You're over thinking this. Write out the corresponding reactions.
First, it can donate a proton as in
HPO4^2- ==> H^+ + PO4^3-

And it can accept a proton.
HPO4^2- + H^+ ==> H2PO4^-

That's all there is to it. You add a proton or you take away a proton.