Chemistry · Coordination compounds · chapter 5 of 10

Class 12 Chemistry Chapter 5: Coordination Compounds case study question with answers

One case-based question of the kind the board asks: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

Three solid samples, P, Q and R, have the same composition, CrCl₃·6H₂O, but differ in colour. In each, chromium has coordination number 6 and every water molecule not bound to chromium sits in the crystal lattice. A student dissolves 0.010 mol of each in water, adds excess AgNO₃(aq) at once, and weighs the AgCl precipitated.

Sample	Colour	AgCl precipitated / mol
P	violet	0.030
Q	grey-green	0.020
R	dark green	0.010

Part 1 · 1 mark

The formula of Q is

(a) [Cr(H₂O)₆]Cl₃(b) [Cr(H₂O)₅Cl]Cl₂·H₂O(c) [Cr(H₂O)₄Cl₂]Cl·2H₂O(d) [Cr(H₂O)₃Cl₃]·3H₂O

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Answer: (b) [Cr(H₂O)₅Cl]Cl₂·H₂O

0.020 mol AgCl from 0.010 mol means two ionisable Cl⁻ per formula unit; the third Cl is bound, with five H₂O, and the sixth H₂O is in the lattice.

Part 2 · 1 mark

The numbers of ions given per formula unit in solution by P, Q and R are, in order,

(a) 4, 3, 2(b) 3, 2, 1(c) 2, 3, 4(d) 4, 2, 1

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Answer: (a) 4, 3, 2

P, [Cr(H₂O)₆]³⁺ + 3Cl⁻: 4 ions; Q, [Cr(H₂O)₅Cl]²⁺ + 2Cl⁻: 3; R, [Cr(H₂O)₄Cl₂]⁺ + Cl⁻: 2. Option b counts only the chloride ions.

Part 3 · 2 marks

Name the type of isomerism shown by P, Q and R, and explain why the same composition gives different amounts of AgCl.

Answer and marking scheme

They are solvate (hydrate) isomers: they differ in whether water is bound to chromium or held free in the lattice. Water that enters the coordination sphere pushes chloride out of it, so the number of ionisable Cl⁻ outside the square bracket, the only chloride that AgNO₃ precipitates at once, changes from 3 to 2 to 1.

solvate (hydrate) isomerism: water bound to the metal or free in the lattice (1)
only Cl⁻ outside the coordination sphere is precipitated; bound water replaces bound Cl⁻ (1)

The cation of R can exist in two geometrical forms. Name them, describe the position of the two Cl atoms in each, and state whether either is optically active.

Answer and marking scheme

[Cr(H₂O)₄Cl₂]⁺ is octahedral [MX₂L₄]: in cis the two Cl are adjacent (90°), in trans opposite (180°). Both have a mirror plane through the two Cl atoms and the metal, so neither is optically active.

cis (Cl at 90°) and trans (Cl at 180°) (1)
neither is optically active: each has a mirror plane (1)

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