Experiment 125

Objective:

To identify the chemical structure and formula of an unknown salt sample (Al₂(SO₄)₃) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be aluminum sulfate (Al₂(SO₄)₃), is identified based on a series of tests targeting its cation (basic group) and anion (acidic group). These tests will confirm the chemical composition and properties of the salt.

Sample Physical Characteristics:

·        Color: White

·        Physical State: Crystalline

·        Solubility: Soluble in cool water

Materials:

Chemicals:

·        Unknown salt sample (Al₂(SO₄)₃)

·        Distilled water

·        Hydrochloric acid (HCl)

·        Hydrogen sulfide gas (H₂S)

·        Ammonium chloride (NH₄Cl)

·        Ammonium hydroxide (NH₄OH)

·        Ammonium carbonate ((NH₄)₂CO₃)

·        Sodium hydrogen phosphate (Na₂HPO₄)

·        Freshly prepared ferrous sulfate (FeSO₄)

Apparatus:

·        Test tubes and holder

·        Bunsen burner

·        Thermometer

·        500 mL flask

·        Test tube stand

·        Pipette

Reagents Preparation:

1.    Dissolve 1.275 g of the unknown salt in 50 mL of distilled water using a 500 mL flask.

2. Prepare 5% HCl and ammonium hydroxide solutions in separate beakers.

3.    Generate hydrogen sulfide gas using a round-bottom flask.

4. Prepare 5% ferrous sulfate solution in a 250 mL flask.

Procedure:

1.    Basic Group (Cation) Tests:

·        Step 1: Add dilute HCl to the salt sample in a test tube. No precipitation indicates the absence of Pb²⁺ ions.

·        Step 2: Heat the solution and pass H₂S gas through it. No precipitation forms, ruling out the presence of Pb²⁺ and Cu²⁺ ions.

·        Step 3: Heat the solution to expel H₂S gas. Add NH₄Cl and NH₄OH to make the solution basic. A white jelly-like precipitate forms, suggesting the presence of Fe²⁺ or Al³⁺ ions.

·        Step 4: Flow H₂S through the basic solution again. No precipitation confirms the absence of Ni²⁺, Co²⁺, and Zn²⁺ ions.

·        Step 5: Reheat the solution to remove H₂S gas. Add NH₄OH, NH₄Cl, and (NH₄)₂CO₃. No precipitation indicates Ba²⁺ and Ca²⁺ are absent.

·        Step 6: Divide the solution into two portions:

·        Portion 1: Heat and add NH₄OH and Na₂HPO₄. No precipitation indicates Mg²⁺ is absent.

·        Portion 2: Heat and evaporate the solution. The absence of any solid residue suggests Na⁺ and K⁺ are not present.

2.    NH₄⁺ Radical Test:

·       Add Nelson reagent to a test tube containing the sample solution. The absence of brown precipitation confirms that NH₄⁺ is not present.

3.    Al³⁺ Confirmation Test:

·       Dissolve the white jelly-like precipitate in water. Add NaOH to form a solution, confirming the presence of Al³⁺.

·       Confirmation Test: Add ammonium chloride to the solution and heat. The white precipitate reforms, further confirming Al³⁺.

4.    Acidic Group (Anion) Tests:

·       Step 1: Add nitric acid followed by silver nitrate to the sample solution. No color change or precipitation indicates the absence of Cl⁻, Br⁻, and I⁻ ions.

·       Step 2: Add sodium nitroprusside to another portion. No violet color indicates the absence of S²⁻ ions.

·       Step 3: Add dilute HCl and barium chloride. White precipitate formation confirms the presence of SO₄²⁻ ions.

·       Step 4: Ring Test: Add concentrated sulfuric acid to the solution. Cool it and carefully layer ferrous sulfate on top. The absence of a brown ring confirms that NO₃⁻ is not present.

Observations:

·      Cation Identified: Al³⁺ (aluminum ion)

·      Anion Identified: SO₄²⁻ (sulfate ion)

Thus, the unknown salt is identified as Aluminum Sulfate (Al₂(SO₄)₃).

Discussion Questions:

1.    What is aluminum sulfate?

2.    What happens when H₂S is added to an aluminum sulfate solution?

3.    What color forms when aluminum hydroxide is present in solution?

4.    What happens when NH₄OH and NH₄Cl are added to Al₂(SO₄)₃ solution?