A-Level Chemistry October/November 2024 Q4(b)(i): Deduce the formula of complex ion G. Include its overall charge. formula of G

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  To deduce the formula of complex ion G, we must identify all its components and their respective charges to calculate the overall charge.

  1. Central Metal Ions and Ligands:

     • The complex contains two cobalt (Co) ions.
     • There are eight ammonia (NH₃) ligands.
     • There is one peroxo (O₂²⁻) bridging ligand.
     • There is one amido (NH₂⁻) bridging ligand.

  2. Oxidation States and Charges:

     • Cobalt is in the +3 oxidation state, so two Co³⁺ ions contribute a total charge of $2 \times (+3) = +6$.
     • Ammonia (NH₃) is a neutral ligand, so eight NH₃ ligands contribute a charge of $8 \times 0 = 0$.
     • The peroxo ligand (O₂²⁻) has a charge of -2.
     • The amido ligand (NH₂⁻) has a charge of -1.

  3. Overall Charge Calculation:

     • Overall charge = (charge from Co) + (charge from NH₃) + (charge from O₂²⁻) + (charge from NH₂⁻)
     • Overall charge = $(+6) + (0) + (-2) + (-1) = +3$.

  4. Final Formula:

     • The formula is written with the metal ions first, followed by the ligands, all enclosed in square brackets with the overall charge as a superscript.

  Formula of G:

  [Co₂O₂NH₂(NH₃)₈]³⁺

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  How the marks are awarded

  • B1 — Correctly identifying at least two of the three key components: the core atomic arrangement (Co₂O₂NH₂), the number of ammonia ligands (8), and the final correct charge (+3).
  • B1 — Correctly identifying all three components, leading to the complete and correct final formula [Co₂O₂NH₂(NH₃)₈]³⁺, including the square brackets and the correct overall charge.

  Common mistakes

  • Incorrectly calculating the overall charge, often by assigning the wrong charge to the peroxo (e.g., treating it as two O²⁻ oxides, -4) or amido (e.g., treating it as neutral, 0) ligands.
  • Forgetting to include the overall charge (3+) or placing it inside the square brackets.
  • Writing the formula with incorrect stoichiometry, for example miscounting the number of cobalt ions or ammonia ligands.
  • Omitting the square brackets required for the formula of a complex ion.

  Examiner tip: Always determine the overall charge of a complex ion by systematically summing the known oxidation state of the central metal ion(s) and the charges of all individual ligands.

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