Experiement 140

Objective:

Analyze and identify the chemical structure and formula of an unknown salt sample, magnesium chloride [MgCl₂], through basic and acidic group tests.

Introduction:

The unknown salt sample, magnesium chloride, will be identified through tests on both its cation (basic group) and anion (acidic group). These tests will reveal its chemical structure and formula.

Sample Characteristics:

·        Color: White

·        Physical State: Crystalline

·        Solubility: Soluble in cool water

Materials for the Experiment:

Chemicals:

·        Supplied sample: MgCl₂

·        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

·        Test tube holder

·        Bunsen burner

·        Thermometer

·        500 ml flask

·        Test tube stand

·        Pipette

Reagents Preparation:

·        Dissolve 1.275 g of the supplied salt sample in 50 ml distilled water in a 500 ml flask.

·        Prepare 5% HCl in a beaker.

·        Prepare ammonium hydroxide and ammonium chloride solutions in separate beakers.

·        Produce hydrogen sulfide gas in a round-bottom flask.

·        Prepare 5% ferrous sulfate solution in a 250 ml flask.

Procedure:

1.    Basic Group (Cation) Test:

·        Take the sample in a test tube and add dilute HCl. Observation: No precipitate forms, indicating the absence of Pb²⁺.

·        Heat the solution and pass H₂S gas through it. Observation: No black precipitate forms, confirming the absence of Pb²⁺ and Cu²⁺.

·        Heat the solution to remove H₂S gas, and add NH₄Cl and NH₄OH to create a basic solution. Observation: No precipitate forms, ruling out Fe²⁺ and Al³⁺.

·        Pass H₂S gas again through the basic solution. Observation: No precipitate forms, confirming the absence of Ni²⁺, Co²⁺, and Zn²⁺.

·        Heat the solution again to remove H₂S gas, and add NH₄OH, NH₄Cl, and (NH₄)₂CO₃. Observation: No precipitate forms, ruling out Ba²⁺ and Ca²⁺.

·        Divide the solution into two portions:

1.    Heat the first portion and add NH₄OH and Na₂HPO₄. Observation: A white precipitate forms, indicating the presence of Mg²⁺.

2.    Heat and evaporate the second portion. Observation: No residue, confirming the absence of Na⁺ and K⁺.

2.    NH₄⁺ Radical Test:

·       Add Nessler's reagent to the main sample solution in a test tube. Observation: No brown precipitate forms, indicating the absence of NH₄⁺.

3.    Acidic Group (Anion) Test:

·       Add nitric acid and a small amount of silver nitrate to the sample in a test tube. Observation: A white precipitate forms, indicating the presence of Cl⁻, Br⁻, or I⁻.

·       Dissolve the white precipitate in ammonium hydroxide. Observation: The precipitate dissolves, confirming the presence of Cl⁻.

·       Add sodium nitroprusside to the main solution. Observation: No violet color forms, ruling out S²⁻.

·       Add dilute HCl and barium chloride to the main solution. Observation: No white precipitate forms, ruling out SO₄²⁻.

·       Perform a ring test: Add concentrated sulfuric acid to the main solution, cool, and carefully add freshly prepared ferrous sulfate to form a separate layer. Observation: No brown ring forms between the layers, confirming the absence of NO₃⁻.

Observations:

·       Cation: Mg²⁺ (Magnesium)

·       Anion: Cl⁻ (Chloride)

Conclusion: The unknown salt is magnesium chloride, [MgCl₂].

Questions:

1.    What is magnesium chloride?

2.    What happens when NH₄OH and Na₂HPO₄ are added to magnesium salt?

3.    What color forms when NH₄OH and Na₂HPO₄ are added to magnesium salt?