The sketch graph for the titration of ethanoic acid, CH₃CO₂H, with sodium hydroxide is shown. [Figure showing titration curve, pH vs volume of NaOH, with a circled region and "equivalence point" labelled] In the region circled on the graph, identify the two organic species that are present in the solution. Explain why the pH of the mixture only changes slowly and gradually in this region when sodium hydroxide is being added.

1. (a)(i)The sketch graph for the titration of ethanoic acid, CH₃CO₂H, with sodium hydroxide is shown. [Figure showing titration curve, pH vs volume of NaOH, with a circled region and "equivalence point" labelled] In the region circled on the graph, identify the two organic species that are present in the solution. Explain why the pH of the mixture only changes slowly and gradually in this region when sodium hydroxide is being added.[3]
2. (a)(ii)The equivalence point in this acid-base titration is where the two solutions have been mixed in exactly equal molar proportion. Suggest why the pH is greater than 7 at the equivalence point in this titration.[1]
3. (b)An impure sample of ammonium vanadate(V), NH₄VO₃, with mass 0.150g, is dissolved in an excess of dilute acid. In this solution all vanadium is present as VO₂⁺ ions. An excess of zinc powder is added to the solution and all the VO₂⁺ ions are reduced to V²⁺ ions. The mixture is filtered to remove any remaining zinc powder. VO₂⁺ + 4H⁺ + 3e⁻ → V²⁺ + 2H₂O When the resulting solution is titrated, 20.10 cm³ of 0.0250 mol dm⁻³ acidified MnO₄⁻ oxidises all V²⁺ ions back to VO₂⁺ ions. MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O Calculate the percentage by mass of NH₄VO₃ in the 0.150g impure sample of NH₄VO₃. Give your answer to three significant figures. [Mr: NH₄VO₃, 116.9][3]

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