Question

Which of the following is called alum? (1) \(\left(\mathrm{NII}_{4}\right) \mathrm{SO}_{4} \cdot \mathrm{FeSO}_{4} \cdot 6 \mathrm{II}_{2} \mathrm{O}\) (2) \(\mathrm{Na}_{2} \mathrm{SO}_{4} \cdot \Lambda \mathrm{l}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{II}_{2} \mathrm{O}\) (3) \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{II}_{2} \mathrm{O}\) (4) \(\mathrm{Na} \mathrm{AlO}_{2}\)

Short answer

Option 2: \[ \text{Na}_2\text{SO}_4 \cdot \text{Al}_2(\text{SO}_4)_3 \cdot 24H_2O \] is called alum.

Step-by-step solution

- Understand the Definition of Alum

Alum is a type of chemical compound, usually a hydrated double sulfate salt of aluminum with the general formula \[ M \text{I}_2SO_4(M_III)_2(SO_4)_3 \cdot 24 \text{H}_2O \], where M can be a monovalent cation such as potassium or sodium, and M can be a trivalent cation like aluminum.

- Analyze the Given Options

Look at the given options and match them with the general formula of alum. Identify which option fits the definition.

- Check Each Option

Option 1: \[ (NH_4)SO_4 \cdot FeSO_4 \cdot 6H_2O \] \( \text{does not match the definition}. \) Option 2: \[ \text{Na}_2\text{SO}_4 \cdot \text{Al}_2(\text{SO}_4)_3 \cdot 24H_2O \] \( \text{matches the definition of alum}. \) Option 3: \[ KCl \cdot MgCl_2 \cdot 6H_2O \] \( \text{does not match the definition.} \) Option 4: NaAlO2 \( \text{does not match the definition.} \)

- Identify the Correct Answer

Based on the analysis, the compound that matches the definition of alum is Option 2: \[ \text{Na}_2\text{SO}_4 \cdot \text{Al}_2(\text{SO}_4)_3 \cdot 24H_2O \].

Key concepts

hydrated double sulfate salts

Hydrated double sulfate salts are special types of chemical compounds. These compounds contain two different sulfate salts combined in a specific ratio. They also include water molecules as part of their solid structure. These water molecules are known as water of hydration. For example, alums are a common type of hydrated double sulfate salt.
A typical formula for these salts is \text{M}_2\text{SO}_\({4}\) \text{M}_{\(III\)}_2(\text{SO}_4)_3 \textto 24\text{H}_2\text{O}, where 'M' is a monovalent cation, and '\(M_{III}\)' is a trivalent cation.

• Monovalent cations: Potassium (K\text{⁺}), Sodium (Na\text{⁺})
• Trivalent cations: Aluminum (Al\text{³⁺}), Iron (Fe\text{³⁺})

Understanding the structure of these salts can help identify whether a compound is an alum by checking for specific cations and the presence of water of hydration.

chemical compound definitions

Chemical compounds are substances formed from two or more elements chemically bonded together. They can be categorized based on their structures and properties. A key aspect is understanding how elements combine to form compounds.
There are several types of compounds, including:

• Ionic compounds: Formed from the electrostatic attraction between positive and negative ions (e.g., NaCl)
• Covalent compounds: Formed by the sharing of electrons between atoms (e.g., H₂O)
• Metallic compounds: Formed by the pooling of electrons shared among metal atoms (e.g., Fe, Cu)

For the context of alums and hydrated double sulfate salts, compounds must match specific formulas and include certain elements arranged in a defined way. For instance, in the provided exercise, only option 2 fits the definition of an alum because it involves the proper arrangement of cations and anions with definite water of hydration. Understanding these definitions assists in accurately categorizing and identifying compounds.

alum formula analysis

Analyzing the formula of a compound to determine if it's an alum involves checking if it fits the alum structure. This consists of identifying the presence of specific ions and the correct amount of water of hydration.
Steps for formula analysis:
1. Check for monovalent cation (\text{M}): Common examples are potassium (K\text{⁺}) and sodium (Na\text{⁺}).
2. Check for trivalent cation (\text{M}_{\(III\)}): Common examples are aluminum (Al\text{³⁺}) and iron (Fe\text{³⁺}).
3. Verify the sulfate components (\text{SO}₄) are present in specific ratios.
4. Ensure the compound includes 24 water molecules as water of hydration.
The exercise options were analyzed using this framework. For example:

• Option 1: Missing the proper amount of water of hydration.
• Option 3: Incorrect ions and lacks water of hydration.
• Option 4: Not related to the alum structure at all.

Only Option 2, \text{Na}₂\text{SO}₄ \text{Al}₂(\text{SO}₄)₃ \textto 24\text{H}_2\text{O}, matches all the necessary criteria, confirming it as the alum.