How to find out which is acid, and which is base in an equation like this- (H2PO4)- + HF ---->

Answer 1

Answer 2

If you have H₂PO₄^- + HF and you want to determine which reactant is the acid, and which is the base, you can look at which one will donate a proton (H+) and which one will accept that proton.

HF is an acid since it can donate a proton, but it cannot accept a proton. HF ==> H⁺ + F^-

H₂PO₄^- can also be an acid as it can also donate a proton. H₂PO₄^- ==> H⁺ + HPO₄^2-. However, it can also accept a proton. H₂PO₄^- + H⁺ ==> H₃PO₄. So, it is amphoteric.

So, in the reaction presented, it would be feasible to write

H₂PO₄^- + HF ==> H₃PO₄ + F^-

You can also look up the Ka values for H2PO4 and HF and find that HF is a much stronger weak acid than is H2PO4.

So, the answer to the question is

H₂PO₄^- is the base

HF is the acid