Experiment 129

Objective:

To identify the chemical structure and formula of an unknown salt sample (CuCl₂) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be copper chloride (CuCl₂), is identified based on a series of tests targeting its cation (basic group) and anion (acidic group). These tests will confirm the chemical composition and properties of the salt.

Sample Physical Characteristics:

Color: Blue-green

Physical State: Crystalline

Solubility: Soluble in cool water

Materials:

Chemicals:

·        Unknown salt sample (CuCl₂)

·        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 and holder

·        Bunsen burner

·        Thermometer

·        500 mL flask

·        Test tube stand

·        Pipette

Reagents Preparation:

1.    Dissolve 1.275 g of the unknown salt in 50 mL of distilled water using a 500 mL flask.

2.    Prepare 5% HCl and ammonium hydroxide solutions in separate beakers.

3.    Generate hydrogen sulfide gas using a round-bottom flask.

4.    Prepare 5% ferrous sulfate solution in a 250 mL flask.

Procedure:

1.    Basic Group (Cation) Tests:

·        Step 1: Add dilute HCl to the salt sample in a test tube. No precipitation indicates the absence of Pb²⁺ ions.

·        Step 2: Heat the solution and pass H₂S gas through it. The formation of black precipitation suggests the presence of Pb²⁺ or Cu²⁺ ions. This will be confirmed in later experiments.

·        Step 3: Heat the solution to expel H₂S gas. Add NH₄Cl and NH₄OH to make the solution basic. No precipitation indicates Fe²⁺ and Al³⁺ are absent.

·        Step 4: Flow H₂S through the basic solution again. No precipitation confirms the absence of Ni²⁺, Co²⁺, and Zn²⁺ ions.

·        Step 5: Reheat the solution to remove H₂S gas. Add NH₄OH, NH₄Cl, and (NH₄)₂CO₃. No precipitation indicates Ba²⁺ and Ca²⁺ are absent.

·        Step 6: Divide the solution into two portions:

·       Portion 1: Heat and add NH₄OH and Na₂HPO₄. No precipitation indicates Mg²⁺ is absent.

·        Portion 2: Heat and evaporate the solution. The absence of any solid residue suggests Na⁺ and K⁺ are not present.

2.    NH₄⁺ Radical Test:

·       Add Nelson reagent to a test tube containing the sample solution. The absence of brown precipitation confirms that NH₄⁺ is not present.

3.    Cu²⁺ Confirmation Test:

·       Dissolve any black precipitate in water, forming a blue solution, indicating the presence of Cu²⁺ ions.

·       Confirmation Test 1: Add potassium chromate to the blue solution. Yellow precipitation occurs, which dissolves in NaOH, confirming Cu²⁺.

·       Confirmation Test 2: Add NH₄OH to another portion. The initial blue precipitate turns into a deep blue solution with excess NH₄OH.

4.    Acidic Group (Anion) Tests:

·  Step 1: Add nitric acid followed by silver nitrate to the sample solution. White precipitation occurs, indicating the presence of Cl⁻, Br⁻, or I⁻.

·  Step 2: Dissolve the white precipitate in ammonium hydroxide. This confirms the presence of Cl⁻.

·  Step 3: Add sodium nitroprusside to another portion. No violet color indicates the absence of S²⁻ ions.

·  Step 4: Add dilute HCl and barium chloride. No precipitation indicates SO₄²⁻ is absent.

·  Step 5: Ring Test: Add concentrated sulfuric acid to the solution. Cool it and carefully layer ferrous sulfate on top. The absence of a brown ring confirms that NO₃⁻ is not present.

Observations:

·        Cation Identified: Cu²⁺ (cupric ion)

·        Anion Identified: Cl⁻ (chloride ion)

Thus, the unknown salt is identified as Copper Chloride (CuCl₂).

Discussion Questions:

1.    What is copper chloride?

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

3.    What color forms when copper hydroxide is present in solution?

4.    What happens when NH₄OH is added to the solution?