Experiment 130

Objective:

To identify the chemical structure and formula of an unknown salt sample (CuSO₄) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be copper sulfate (CuSO₄), is identified based on a series of tests targeting its cation (basic group) and anion (acidic group). By conducting these tests, the chemical composition and properties of the salt can be accurately determined.

Sample Physical Characteristics:

·       Color: Blue

·       Physical State: Crystalline

·       Solubility: Soluble in cool water

Materials:

Chemicals:

·       Unknown salt sample (CuSO₄)

·       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:

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

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

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

·       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.

·       Step 3: Heat the solution to expel H₂S. 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.

Cu²⁺ Confirmation Test:

·       Dissolve the 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.

2.    Acidic Group (Anion) Tests:

·       Step 1: Add nitric acid followed by silver nitrate to the sample solution. The absence of precipitation indicates Cl⁻, Br⁻, and I⁻ are not 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. 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 indicates NO₃⁻ is not present.

Observations:

Cation Identified: Cu²⁺ (cupric ion)

Anion Identified: SO₄²⁻ (sulfate ion)

Thus, the unknown salt is identified as Copper Sulfate (CuSO₄).

Discussion Questions:

·       What is copper sulfate?

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

·       What color forms when copper hydroxide is present in solution?

·       What happens when NH₄OH is added to the solution?