Experiment 177

Objective:

Analyze and identify the chemical structure and formula of an unknown salt sample, barium chloride [BaCl₂], through basic and acidic group tests.

Introduction:

The unknown salt sample, barium chloride, will be identified through tests on its cation (basic group) and anion (acidic group). These tests 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: BaCl₂

●       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, indicating the absence of Pb²⁺.

●      Heat the solution and pass H₂S gas through it. Observation: No precipitate, 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 brown jelly-like precipitate forms, ruling out Fe²⁺ and Al³⁺.

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

●      Heat the solution again to remove H₂S gas, and add NH₄OH, NH₄Cl, and (NH₄)₂CO₃. Observation: A white precipitate forms, suggesting the presence of Ba²⁺ or Ca²⁺.

●      Divide the solution into two portions:

1.    Heat the first portion and add NH₄OH and Na₂HPO₄. Observation: No precipitate, ruling out 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, indicating the absence of NH₄⁺.

3.    Confirmation Tests:

●      Dissolve the white precipitate in dilute acetic acid and heat. Observation: The precipitate dissolves completely, confirming the presence of Ba²⁺ or Ca²⁺.

●      Add potassium chromate solution to a new portion of the solution. Observation: A yellow precipitate forms, confirming the presence of Ba²⁺.

●      Dissolve the yellow precipitate in dilute HCl, then add sulfuric acid. Observation: A white precipitate forms, further confirming the presence of Ba²⁺.

4.    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 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: Ba²⁺ (Barium)

●      Anion: Cl⁻ (Chloride)

Conclusion: The unknown salt is barium chloride, [BaCl₂].

Questions:

1.    What is barium chloride?

2.    What color forms when potassium chromate is added to a barium chloride solution?