Question:

Complex A has a composition of \(\mathrm{H_{12}O_6Cl_3Cr}\). If the complex on treatment with conc.\(\mathrm{H_2SO_4}\) loses 13.5% of its original mass, the correct molecular formula of A is :
[Given: atomic mass of Cr = 52 amu and Cl = 35 amu]

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Conc. H2SO4 removes only water of crystallisation (lattice water), not coordinated water. Find the molar mass and see how many H2O correspond to 13.5%.
Updated On: Nov 5, 2026
  • \(\mathrm{[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O}\)
  • \(\mathrm{[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O}\)
  • \(\mathrm{[Cr(H_2O)_3Cl_3] \cdot 3H_2O}\)
  • \(\mathrm{[Cr(H_2O)_6]Cl_3}\)
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The Correct Option is B

Solution and Explanation

Molar mass of A = \(\mathrm{CrCl_3 \cdot 6H_2O}\) = 52 + 3 x 35 + 6 x 18 = 52 + 105 + 108 = 265 g/mol.
Mass lost = 13.5% of 265 = 35.8 g, which is about 36 g = 2 x 18 g.
So the complex contains 2 molecules of water of crystallisation (removed by conc. H2SO4), and the other 4 H2O are coordinated.
With 3 Cl in total and 6 coordination sites: 4 H2O + 2 Cl as ligands + 1 Cl as counter ion.
Formula: \(\mathrm{[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O}\).
Answer: (2) [Cr(H2O)4Cl2]Cl.2H2O
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