posted by Amber .
An oxalic acid solution contains the following species in varying concentrations: H2C2O4, HC2O4-, C2O4^2-, and H+. Which of the above four species can act only as acids, which can act only as bases, and which can act as both acids and bases?
You need to apply the definition. I assume you are using the Bronsted-Lowry theory. Acids donate a proton; bases accept a proton.
H2C2O4, for example, can ONLY donate a proton (you know there is no such thing as H3C2O4 or H4C2O4) so it looks like this. H2C2O4 ==> H^+ + HC2O4^-. Another for example is that HC2O4^- can act as both an acid and a base. WHY? Because it can donate a H^+ as in HC2O4^- ==> H^+ + C2O4^2- (the fact that H^+ and C2O4^2- are listed in the same question should tell you that those exist so it can go to the right and act as an acid OR it can do HC2O4^- + H^+ ==> H2C2O4 and act as a base.) That takes care of two of them; I'll leave the other two for you.