A six-coordinate complex of formula CrCl3.6H2O has green color. A 0.1 M solution of the complex, when treated with an excess of AgNO3, gave 28.7 g of a white precipitate. The formula of the complex would be_.
Correct answer: B. [Cr(H2O)5Cl]Cl2.2H2O
- A. [Cr(H2O)6]Cl3
- B. [Cr(H2O)5Cl]Cl2.2H2O
- C. [Cr(H2O)4Cl2]Cl.2H2O
- D. [Cr(H2O)3Cl3].3H2O
Explanation
The answer is B) Cr(H2O)5Cl]Cl2.2H2O. The complex has a total of 6 ligands, 6 water molecules and 1 chloride ion. When the complex is treated with AgNO3, the chloride ions will react with the Ag+ ions to form AgCl precipitate. The mass of the precipitate is 28.7 g, which is equivalent to 0.2 moles of AgCl. Since each mole of AgCl contains 1 mole of chloride ion, the complex must contain 0.2 moles of chloride ions. This means that there are 2 chloride ions in the complex, which are coordinated to the central Cr atom. The other 4 ligands are water molecules. Therefore, the formula of the complex is Cr(H2O)5Cl]Cl2.2H2O. The other options are incorrect because they do not have the correct number of chloride ions. Option A has 3 chloride ions, option C has 4 chloride ions, and option D has 6 chloride ions.
This question appeared on the NUMS 2019 (cancelled sitting) paper, which you can sit online with every answer explained.
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