3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.
Question 1Level 2 · ExamSingle correct
Chromium(III) chloride hexahydrate, CrCl3⋅6H2O, exists as several hydrate isomers. In each, Cr(III) is octahedral (coordination number 6) and the only possible ligands are H2O and Cl−. A sample of one isomer X containing 0.0100 mol of Cr is dissolved in cold water and treated at once with excess AgNO3(aq); 2.87 g of AgCl precipitates immediately. When solid X is kept over concentrated H2SO4, it loses 1 mol of H2O per mol of X (only water not bonded to Cr is lost). (Ag = 108, Cl = 35.5) The complex cation present in X is ______. Exactly one option is correct.
A[Cr(H2O)6]3+
B[Cr(H2O)4Cl2]+
C[Cr(H2O)5Cl]2+
D[Cr(H2O)5Cl]+
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Answer: C
Moles of AgCl =2.87/143.5=0.0200 mol, i.e. 2 mol of free Cl− per mol of Cr. So 2 Cl are outside the coordination sphere (ionisable, primary valence) and 1 Cl is a ligand.
Coordination number 6 then needs 5 H2O ligands; the sixth water is lattice water. Formula: [Cr(H2O)5Cl]Cl2⋅H2O.
Check with the drying data: exactly 1 mol of non-coordinated water is lost. Consistent.
Charge of the cation: +3−1=+2, so the cation is [Cr(H2O)5Cl]2+.
(A) [Cr(H2O)6]3+ would give 3 mol AgCl and lose no water. (B) [Cr(H2O)4Cl2]+ would give 1 mol AgCl and lose 2 mol water. (D) has the wrong charge.
Answer: (C).
Difficulty: HardQuestion 2Level 2 · ExamSingle correct
The IUPAC name of [Co(NH3)4(H2O)Cl]Cl2 is ______. (Use the current IUPAC ligand names ammine, aqua and chlorido.) Exactly one option is correct.
Aaquachloridotetraamminecobalt(III) chloride
Btetraammineaquachloridocobalt(III) chloride
Ctetraammineaquachloridocobaltate(III) chloride
Dtetraammineaquachloridocobalt(II) chloride
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Answer: B
Oxidation state of Co: x+4(0)+0+(−1)=+2 for the cation, and the cation balances 2 Cl−, so x−1=+2, x=+3.
Ligands are listed alphabetically by ligand name, ignoring multiplying prefixes such as tetra-: ammine (a-m) comes before aqua (a-q), which comes before chlorido (c).
The complex is a cation, so the metal name is unchanged (cobalt, not cobaltate).
Name: tetraammineaquachloridocobalt(III) chloride.
(A) alphabetises 'tetraammine' under t: wrong. (C) uses -ate for a cation: wrong. (D) has the wrong oxidation state.
Answer: (B).
Difficulty: HardQuestion 3Level 3 · AdvancedSingle correct
Treat complexes with H2O and F− as high-spin and complexes with CN− as low-spin; all complexes listed are octahedral. Ignore pairing energy in crystal field stabilisation energy (CFSE), which is expressed in units of Δo. Which one of the following pairs has the same spin-only magnetic moment and also the same CFSE? Exactly one option is correct.
A[Cr(H2O)6]2+ and [Fe(H2O)6]2+
B[Cr(NH3)6]3+ and [Ni(NH3)6]2+
C[Fe(CN)6]3− and [Cu(H2O)6]2+
D[CoF6]3− and [Fe(H2O)6]2+
Show answer and solution
Answer: D
Use t2g at −0.4Δo and eg at +0.6Δo per electron.
(A) [Cr(H2O)6]2+: Cr(II), d4 high spin, t2g3eg1: 4 unpaired, CFSE =−1.2+0.6=−0.6Δo. [Fe(H2O)6]2+: Fe(II), d6 high spin, t2g4eg2: 4 unpaired, CFSE =−1.6+1.2=−0.4Δo. Same moment (4.90 BM), different CFSE.
(B) [Cr(NH3)6]3+: d3, t2g3: 3 unpaired, −1.2Δo. [Ni(NH3)6]2+: d8, t2g6eg2: 2 unpaired, −2.4+1.2=−1.2Δo. Same CFSE, different moment.
(C) [Fe(CN)6]3−: Fe(III) d5 low spin, t2g5: 1 unpaired, −2.0Δo. [Cu(H2O)6]2+: d9, t2g6eg3: 1 unpaired, −2.4+1.8=−0.6Δo. Same moment, different CFSE.
(D) [CoF6]3−: Co(III) d6 high spin, t2g4eg2: 4 unpaired, −0.4Δo; [Fe(H2O)6]2+: same configuration. Both 4.90 BM and −0.4Δo.
Trap: matching only one property (A, B, C each match exactly one).
Answer: (D).
Difficulty: Very hard