Solution: Oxalic acid (H₂C₂O₄) is found in toxic concentrations in rhubarb leaves. The acid forms two ions, HC₂O₄− and C₂O₄2−, by the sequential loss of H + ions. Draw Lewis structures for the three species, and comment on the relative lengths and strengths of their carbon-oxygen bonds. The connections among the atoms are shown below with single bonds only.