Experiment 135

Objective:

To identify the chemical structure and formula of an unknown salt sample (ZnSO₄) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be zinc sulfate (ZnSO₄), 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: Colorless

●       Physical State: Crystalline

●       Solubility: Soluble in cool water

Materials:

Chemicals:

●      Unknown salt sample (ZnSO₄)

●      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 black precipitation indicates the absence of Pb²⁺ and Cu²⁺ ions.

●      Step 3: Heat the solution to expel H₂S gas. Add NH₄Cl and NH₄OH to make the solution basic. No precipitation indicates the absence of Fe²⁺ and Al³⁺ ions.

●      Step 4: Flow H₂S through the basic solution again. The formation of white precipitation suggests the presence of Ni²⁺, Co²⁺, or 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.    Zn²⁺ Confirmation Test:

●      Dissolve the white precipitate in water, add NaOH drop by drop. The white precipitate forms, and upon adding excess NaOH, the precipitate dissolves, confirming the presence of Zn²⁺.

●      Confirmation Test 1: Add acetic acid and pass H₂S gas through the solution. White precipitate forms, confirming Zn²⁺.

●      Confirmation Test 2: Add acetic acid to the solution and then potassium ferrocyanide. White precipitate confirms the presence of Zn²⁺.

4.    Acidic Group (Anion) Tests:

●      Step 1: Add nitric acid followed by silver nitrate to the sample solution. No precipitation indicates the absence of Cl⁻, Br⁻, and I⁻ ions.

●      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. White precipitate formation confirms the presence of SO₄²⁻ ions.

●      Step 4: 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: Zn²⁺ (zinc ion)

●      Anion Identified: SO₄²⁻ (sulfate ion)

Thus, the unknown salt is identified as Zinc Sulfate (ZnSO₄).

Discussion Questions:

1.    What is zinc sulfate?

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

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

4.    What happens when potassium ferrocyanide is added to the solution?