Final answer:
The correct order of species from highest to lowest concentration at equilibrium is (c) H₂C₂O₄ > HC₂O₄⁻ > C₂O₄²⁻ > H₃O⁺.
Step-by-step explanation:
In an aqueous solution of the weak acid H₂C₂O₄, the four species present at equilibrium are H₃O⁺, HC₂O₄⁻, C₂O₄²⁻, and H₂C₂O₄. To identify the species in order of highest to lowest concentration at equilibrium, we need to understand the ionization behavior of H₂C₂O₄.
H₂C₂O₄ is a weak acid, which means it does not ionize completely in water. The dominant species in the solution will be the unionized form, H₂C₂O₄. HC₂O₄⁻ and C₂O₄²⁻ are the ionized forms of the acid, but since H₂C₂O₄ is a weak acid, the concentration of these ions will be lower compared to the unionized form. Finally, H₃O⁺ ions will be present in the solution due to the ionization of water itself.
Therefore, the correct order of species from highest to lowest concentration at equilibrium is H₂C₂O₄ > HC₂O₄⁻ > C₂O₄²⁻ > H₃O⁺.