Experiment 128

Objective:

To identify the chemical structure and formula of an unknown salt sample (Cu(NO₃)₂) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be copper nitrate (Cu(NO₃)₂), 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 (Cu(NO₃)₂)

●       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. No precipitation forms, which could indicate the possible presence of Cu²⁺. 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. No white precipitation indicates Cl⁻, Br⁻, and I⁻ are present.

  Step 2: Add sodium nitroprusside to another portion. No violet color indicates the absence of S²⁻ ions.

  Step 3: Add dilute HCl and barium chloride. No precipitation indicates SO₄²⁻ is absent.

  Step 4: Ring Test: Add concentrated sulfuric acid to the solution. Cool it and carefully layer ferrous sulfate on top. A brown ring forms between the layers, confirming the presence of NO₃⁻.

Observations:

●       Cation Identified: Cu²⁺ (cupric ion)

●       Anion Identified: NO₃⁻ (nitrate ion)

Thus, the unknown salt is identified as Copper Nitrate (Cu(NO₃)₂).

Discussion Questions:

1.What is copper nitrate?

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?