The chemical formula of oxalic is [tex]H_{2}C_{2}O_{4}[/tex]
When oxalic acid reacts with water, first, oxalic acid removes one proton and results in the formation of mono acids.
After that, in second step, oxalic acid in aqueous solution removes another proton which shows it is a polyprotic acid.
The chemical equations are: (the reactions occurs in two steps due to presence to hydrogen atoms).
When one proton is removed:
[tex]H_{2}C_{2}O_{4} (aq)+H_{2}O (l)\rightarrow HC_{2}O_{4}^{-}(aq)+H_{3}O^{+}(aq)[/tex]
When another proton is removed:
[tex]HC_{2}O_{4}^{-} (aq)+H_{2}O (l)\rightarrow C_{2}O_{4}^{2-}(aq)+H_{3}O^{+}(aq)[/tex]
The dissociation of oxalic acid in water in shown in the image.
