A-LevelChemistryChemical bonding9701/41 · Oct/Nov 2018 · Q27 Marks
Ethanedioate ions, C2O4²⁻, are bidentate ligands. Cr³⁺(aq) and C2O4²⁻(aq) ions form the complex ion [Cr(C2O4)2(H2O)2]⁻. The solubility of calcium ethanedioate, CaC2O4, is 6.65 × 10⁻³gdm⁻³ at 298 K.
- (a)Explain what is meant by the term ligand. [1][1]
- (b)Draw two stereoisomers of this complex ion. You may use [symbol for C2O4²⁻] to represent C2O4²⁻. [2][2]
- (c)(i)Write an expression for the solubility product, Ksp, of CaC2O4. Include its units. Ksp = .......................................................................... units = ................................... [2][2]
- (c)(ii)Calculate the numerical value of Ksp CaC2O4 at 298 K. Give your answer in standard form to two significant figures. Ksp CaC2O4 = ............................................... [2][2]
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Mark scheme
Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.
- [1] species that forms dative bond(s) to a (central) metal atom / ion
- [1] × 2 any two structures
- [1] Ksp = [Ca²⁺][C2O4²⁻]
- [1] units mol² dm⁻⁶
- [1] [Ca²⁺] = [C2O4²⁻] = 6.65 × 10⁻³/128.1 = 5.19 × 10⁻⁵ mol dm⁻³
- [1] Ksp = (5.19 × 10⁻⁵)² = 2.7 × 10⁻⁹ mol² dm⁻⁶
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