Experiment 134

Objective:

To identify the chemical structure and formula of an unknown salt sample (ZnCl₂) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be zinc chloride (ZnCl₂), 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: White

Physical State: Crystalline

Solubility: Soluble in cool water

Materials:

Chemicals:

·        Unknown salt sample (ZnCl₂)

·        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 suggests the absence of Fe²⁺ and Al³⁺ ions.

·       Step 4: Flow H₂S through the basic solution again. The formation of white precipitation indicates 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. White precipitation occurs, confirming the presence of Cl⁻.

·       Step 2: Dissolve the white precipitate in ammonium hydroxide, confirming 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 white precipitate 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: Zn²⁺ (zinc ion)Anion Identified: Cl⁻ (chloride ion)

Thus, the unknown salt is identified as Zinc Chloride (ZnCl₂).

Discussion Questions:

1.    What is zinc chloride?

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

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

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