Experiment 137

Objective:

Analyze and identify the chemical structure and formula of an unknown salt sample, calcium chloride (CaCl2), through basic and acidic group tests.

Introduction:

The unknown salt sample, identified as calcium chloride, will be analyzed using tests for both its cation (basic group) and anion (acidic group). These tests will confirm its chemical structure and formula.

Sample Characteristics:

·       Color: White

·       Physical State: Crystalline

·       Solubility: Soluble in cool water

Materials for the Experiment:

Chemicals:

·       Supplied sample: CaCl2

·       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 tubesTest tube holderBunsen burnerThermometer500 ml flaskTest tube standPipette

Reagents Preparation:

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

2.    Prepare 5% HCl in a beaker.

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

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

5.    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, indicating 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 Ca²⁺ or Ba²⁺.

·       Divide the solution into two portions:

1.    Add ammonium oxalate to the first portion. Observation: A white precipitate of calcium oxalate forms, confirming the presence of Ca²⁺.

2.    Add dilute sulfuric acid to the second portion. Observation: A white precipitate forms, which is soluble in acid, confirming the presence of Ca²⁺.

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: Ca²⁺ (Calcium)

·        Anion: Cl⁻ (Chloride)

Conclusion: The unknown salt is calcium chloride, [CaCl2].

Questions:

1.    What is calcium chloride?

2.    What happens when H2S is added to a calcium salt solution?

3.    What color forms when ammonium oxalate is added to a calcium chloride solution?