A-LevelChemistryAtoms, molecules and stoichiometryFeb/Mar 2021Paper 5 Q220 Marks
Ethanedioic acid is a white crystalline solid. If excess aqueous potassium hydroxide, KOH(aq), is added to dilute ethanedioic acid, H₂C₂O₄(aq), full neutralisation occurs and potassium ethanedioate, K₂C₂O₄(aq), forms. H₂C₂O₄(aq) + 2KOH(aq) → K₂C₂O₄(aq) + 2H₂O(l) If a small amount of potassium hydroxide is added, partial neutralisation takes place and not all H⁺ ions in the acid are replaced by K⁺ ions. Instead an acid salt forms, which crystallises to form a solid with the formula KₐHᵦ(C₂O₄)c•dH₂O. The letters a, b and c represent a ratio of the numbers of species present in the compound and may not necessarily be whole numbers. The relative number of water molecules associated with one formula of the compound is represented by d. A student attempted to determine the values of a, b, c and d in a sample of an acid salt, KₐHᵦ(C₂O₄)c•dH₂O.
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