Experiment 136

Objective:

Analyze and identify the chemical structure and formula of an unknown salt sample, calcium carbonate [CaCO₃], through basic and acidic group tests.

Introduction:

The unknown salt sample, calcium carbonate, will be analyzed through tests on both its cation (basic group) and anion (acidic group). These tests confirm its chemical structure and formula.

Sample Characteristics:

●      Color: White

●      Physical State: Non-crystalline

●      Solubility: Soluble in dilute HCl

Materials for the Experiment:

Chemicals:

●      Supplied sample: CaCO₃

●      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:

1.    Dissolve 1.275 g of the supplied salt sample in 50 ml dilute HCl 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 forms, indicating the absence of Pb²⁺.

●      Heat the solution and pass H₂S gas through it. Observation: A black precipitate forms, indicating the presence of Pb²⁺ or 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: No precipitate forms, ruling out Mg²⁺.

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

2.    Confirmation Tests for Ca²⁺:

●      Take the white precipitate in a test tube, add dilute acetic acid, and heat. Observation: The precipitate dissolves completely, indicating 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 dissolves in acid, confirming Ca²⁺.

3.    Acidic Group (Anion) Test:

●      Add nitric acid and a small amount of silver nitrate to the sample in a test tube. Observation: No white precipitate forms, ruling out Cl⁻, Br⁻, and I⁻.

●      Add sodium nitroprusside to the main solution. Observation: No violet color forms, indicating the absence of 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, confirming the absence of NO₃⁻.

●      Add BaCl₂ to the main solution. Observation: A white precipitate forms, which dissolves in HCl, releasing CO₂ gas as bubbles, confirming the presence of CO₃²⁻.

Observations:

●       Cation: Ca²⁺ (Calcium)

●       Anion: CO₃²⁻ (Carbonate)

Conclusion:  The unknown salt is calcium carbonate, [CaCO₃].

Questions:

1.    What is calcium carbonate?

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

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