Question

Which of the following is the composition of carnallite? (a) \(\mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Fe}_{3} \mathrm{O}_{4}\) (c) \(\mathrm{Na}_{3} \mathrm{AlF}_{6}\) (d) \(\mathrm{Na}_{2} \mathrm{Al}_{2} \mathrm{O}_{3}\)

Short answer

The composition of carnallite is \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \), which corresponds to option (a).

Step-by-step solution

Identify the chemical formula for Carnallite

Carnallite is a hydrous magnesium potassium chloride, and its chemical formula is commonly known as \( \mathrm{KMgCl}_3 \cdot 6 \mathrm{H}_2\mathrm{O} \) or \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This is a solid compound consisting of a mix of chlorides with water.

Analyze Option (a)

Option (a) is \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This matches the known formula for carnallite, containing magnesium, potassium, chloride ions, and water.

Verify other options

Review the other options to ensure they do not match the composition of carnallite: - Option (b): \( \mathrm{Fe}_{3} \mathrm{O}_{4} \) is a form of iron oxide, commonly known as magnetite.- Option (c): \( \mathrm{Na}_{3} \mathrm{AlF}_{6} \) is a sodium aluminum fluoride, known as cryolite.- Option (d): \( \mathrm{Na}_{2} \mathrm{Al}_{2} \mathrm{O}_{3} \) does not correspond to any known compound related to the name provided, it mostly seems like a mixed oxide.

Conclusion

After comparing all given options, it is clear that only option (a) \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \) matches the chemical composition of carnallite. Therefore, option (a) is the correct answer.

Key concepts

Hydrous Magnesium Potassium Chloride

Carnallite, a fascinating mineral, is known for being a hydrous magnesium potassium chloride. This means that its chemical form includes both chlorides of magnesium and potassium along with water molecules. In nature, carnallite often appears as a white or reddish tetrahedral crystalline mineral. Its unique combination of magnesium and potassium chlorides gives it special chemical properties.
One of the essential aspects of understanding carnallite is recognizing its formula:

• The primary formula used is \( \mathrm{KMgCl}_3 \cdot 6 \mathrm{H}_2\mathrm{O} \) or \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \).
• The presence of water in the formula indicates it is a hydrous compound.

Understanding this formula helps to predict the behavior and reactions of carnallite in various chemical processes related to industries and natural formations.

Chemical Formulas in Inorganic Chemistry

Learning about chemical formulas is crucial for anyone studying inorganic chemistry. These formulas represent the composition of compounds, illustrating the types and numbers of atoms involved. When dealing with a compound like carnallite, the formula \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \) provides comprehensive insights into its structure.

• Magnesium Chloride (\(\mathrm{MgCl}_2\)): An ionic compound made up of magnesium and chloride ions.
• Potassium Chloride (\(\mathrm{KCl}\)): Another ionic compound formed by potassium and chloride ions.
• Water (\(\mathrm{H}_2\mathrm{O}\)): Six molecules of water are coordinated in the structure, making it hydrous.

Understanding these individual components helps students identify reactions and interactions that involve these ions, which can be complicated in their interactions but essential for identifying compound characteristics.

Identifying Ionic Compounds

Recognizing ionic compounds involves understanding how their components form and interact. Ionic compounds like carnallite are constructed from positive and negative ions bound together by electrostatic forces. Ionic compounds have distinct characteristics:

• High Melting Points: Due to strong ionic bonds.
• Solubility in Water: Many ionic compounds dissolve in water, breaking into their respective ions.
• Electrical Conductivity: Once dissolved or molten, they can carry an electric current due to free-flowing ions.

The formula \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \) displays these characteristics as it binds magnesium and potassium ions with chloride ions, supplemented with water molecules. Knowing these traits helps in identifying unknown compounds in laboratory settings or in nature, providing a backbone for further analytical chemistry studies.