Experiment 146

Objective:

Analyze and identify the chemical structure and formula of an unknown salt sample, zinc carbonate [ZnCO₃], through basic and acidic group tests.

Introduction:

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

·       Color: White

·       Physical State: Non-crystalline

·       Solubility: Soluble in dilute HCl

Materials for the Experiment:

Chemicals:

·       Supplied sample: ZnCO₃

·       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: 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: A white precipitate forms, indicating the presence of Ni²⁺, Co²⁺, or Zn²⁺.

·       Divide the solution for further confirmation:

1.    Add water and HCl to the white precipitate, heat, and then add NaOH dropwise. Observation: A white precipitate forms, dissolving in excess NaOH, confirming the presence of Zn²⁺.

2.    Add acetic acid to form an acidic solution, then add H₂S gas. Observation: A white precipitate forms, confirming Zn²⁺.

3.    Add acetic acid and then potassium ferrocyanide to the second portion. Observation: A white precipitate forms, indicating Zn²⁺.

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: 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: Zn²⁺ (Zinc)Anion: CO₃²⁻ (Carbonate)

Conclusion:  The unknown salt is zinc carbonate, [ZnCO₃].

Questions:

1.    What is zinc carbonate?

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

3.    What color forms in a zinc hydroxide solution?

4.    What happens when potassium ferrocyanide is added to a zinc carbonate solution?

5.    What happens when calcium carbonate reacts with dilute HCl?