[jcais]

Hello,

I won't be able to see the teacher until next week. I took a test and got only this problem wrong so it is on my mind. It was
What is the conjugate acid of HPO4-2?

I put H+ because isn't it:
(HPO4)2-  (PO4)3- + H+ in the 3rd ionization?

Maybe I am missing something. Can someone please help me?

Thank you.

[Dan]

I think the question is probably referring to

HPO₄^2- + H₃O⁺ <-----> H₂PO₄^- + H₂O

ie, HPO₄^2- is acting as a base rather than an acid.

[sdekivit]

What is the conjugate acid of HPO4-2?

an example:

H₃PO₄ + H₂O <--> H₂PO₄^- + H₃O⁺

now we can say that H₂PO₄^- is the conjugated base of H₃PO₄ and on the other hand we can say that H₃PO₄ is the conjugated acid of H₂PO₄^-

the definition for the conjugated acid: the acid that will be formed when the base dissociates. Thus in the previous example:

H₂PO₄^- + H₂O --> H₃PO₄  + OH^-

but it's actually the same.
now it should be no problem to answer this question