Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Carnalite is (a) \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \).

Step 1: Understanding Carnalite

Carnalite is a naturally occurring mineral. It is known to be a hydrated potassium and magnesium chloride. The name itself helps us identify the elements it contains.

Step 2: Identifying the Composition

Carnalite is composed of magnesium chloride, potassium chloride, and water molecules. In its chemical formula, it is represented as \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This formula indicates that it contains \( \mathrm{MgCl}_{2} \) (magnesium chloride), \( \mathrm{KCl} \) (potassium chloride), with six water molecules.

Step 3: Matching with Options

Now match the known chemical formula of carnalite \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \) with the given options: (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) \( \mathrm{Na}_{2} \mathrm{Al}_{2} \mathrm{O}_{3} \). Option (a) matches exactly with the known composition of carnalite.

Key Concepts

Mineral Identification

When trying to identify a mineral like carnalite, understanding its characteristics and composition is crucial. Minerals are naturally occurring, inorganic solids with a definite chemical composition and crystalline structure. They are identified based on their physical properties, such as color, luster, hardness, streak, and habitat.

In the case of carnalite:

• Color: It is often found in a range of colors from white to reddish-brown.
• Luster: Carnalite typically has a vitreous to greasy luster.
• Hardness: On the Mohs hardness scale, it is relatively soft, rating between 2.5 to 3.
• Streak: The mineral leaves a white streak when rubbed on a streak plate.

By evaluating these characteristics, along with chemical composition, geologists can easily identify carnalite among other minerals found in nature.

Chemical Formula

The chemical formula of a mineral provides a compact and informative way to represent its elemental composition. For carnalite, the chemical formula is expressed as \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). Let’s break this down:

• \( \mathrm{MgCl}_{2} \): This stands for magnesium chloride. It shows that there are two chloride ions for every magnesium ion.
• \( \mathrm{KCl} \): This represents potassium chloride, indicating a one-to-one ratio of potassium and chloride ions.
• \( 6 \mathrm{H}_{2} \mathrm{O} \): This part of the formula indicates that the mineral is hydrated, possessing six water molecules per formula unit.

By analyzing the formula, we understand not only the elements present in carnalite but also their specific ratios, which help in verifying its identity among other minerals.

Hydrated Salt

Hydrated salts are compounds that include a specific number of water molecules within their crystalline structure. These are often termed as "hydrates." The water molecules in hydrated salts play a critical role in their physical characteristics and stability.For carnalite, the formula \( \mathrm{MgCl}_{2} \cdot \mathrm{KCl} \cdot 6 \mathrm{H}_{2} \mathrm{O} \) highlights its identity as a hydrated mineral. The "6 \( \mathrm{H}_{2} \mathrm{O} \)" portion indicates that for every formula unit, there are six associated water molecules. These water molecules are typically bound in such a way that they can be removed through heating, resulting in an anhydrous form of the salt.

Understanding hydrated salts is essential because:

• It affects the mineral's structural integrity and stability.
• It influences how the mineral interacts with the environment, such as its ability to absorb moisture.
• It can change the mineral's weight due to the retained water.

Therefore, acknowledging these aspects of hydrated salts helps in understanding not just the chemical, but also the physical properties of minerals like carnalite.