Complete experimental guide for preparing double salts in chemistry laboratory with detailed procedure and observations
The preparation of double salts represents a fundamental practical in inorganic chemistry laboratories. This experiment demonstrates the formation of complex salts through the combination of two simple salts, creating compounds with distinct properties and enhanced stability. Two common examples include ferrous ammonium sulfate (Mohr's salt) and potash alum, both essential compounds in analytical and industrial applications.
To prepare and crystallize double salts:
And to verify their formation through systematic analysis.
For Mohr's Salt Preparation:
For Potash Alum Preparation:
Double salts are molecular compounds formed when two simple salts crystallize together in stoichiometric proportions. Unlike complex salts, double salts dissociate completely into simple ions when dissolved in water.
Mohr's Salt Formation:
FeSO₄ + (NH₄)₂SO₄ + 6H₂O → (NH₄)₂Fe(SO₄)₂·6H₂OPotash Alum Formation:
K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂OMohr's Salt:
Potash Alum:
Solution Preparation:
Ammonium Sulfate Addition:
Crystallization:
Product Collection:
Solution Preparation:
Crystallization:
Product Collection:
| Parameter | Mohr's Salt | Potash Alum |
|---|---|---|
| Color | Pale green | Colorless |
| Shape | Small crystals | Octahedral crystals |
| Solubility | Soluble in water | Soluble in hot water |
| Weight obtained | ___ g | ___ g |
| Percentage yield | ___% | ___% |
| Odor | Odorless | Odorless |
| Stability | Stable in air | Efflorescent |
The preparation of double salts was successfully accomplished with:
For Mohr's Salt:
For Potash Alum:
A: Double salts are molecular compounds formed by the combination of two simple salts that retain their individual properties in solid state but dissociate completely in solution. Examples include Mohr's salt [(NH₄)₂Fe(SO₄)₂·6H₂O] and potash alum [K₂SO₄·Al₂(SO₄)₃·24H₂O].
A: Concentrated H₂SO₄ prevents the oxidation of Fe²⁺ ions to Fe³⁺ ions and maintains the acidic pH necessary for the stability of the compound.
A: Double salts dissociate completely into simple ions in solution, while complex salts contain coordinate covalent bonds and form complex ions that do not dissociate into simple ions.
A: Slow cooling allows proper arrangement of ions in crystal lattice, resulting in well-defined crystals with better yield and purity.
A: Scratch crystallization provides nucleation sites for crystal formation, promoting rapid and uniform crystal growth.
A: Filtering removes insoluble impurities and undissolved particles that could interfere with crystal formation and affect the purity of the final product.
A: Potash alum crystallizes in octahedral form with a highly symmetrical structure. Each K⁺ ion is surrounded by twelve water molecules, and the compound shows isomorphism with other alums.
A:
Mohr's Salt:
Potash Alum:
This laboratory experiment successfully demonstrates the preparation of double salts through simple crystallization techniques. The formation of Mohr's salt and potash alum illustrates fundamental principles of inorganic chemistry, including stoichiometry, crystallization, and salt formation. Proper technique and attention to experimental conditions yield high-quality crystals suitable for further analysis and practical applications.
The experiment reinforces theoretical concepts while developing essential laboratory skills in weighing, solution preparation, crystallization, and product characterization. Both double salts prepared show characteristic properties and find extensive use in analytical chemistry and industrial processes.
This comprehensive guide provides all essential information for successfully conducting the double salt preparation experiment in educational laboratory settings. Students should follow all safety protocols and seek instructor guidance when necessary.
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