Experiment 131

Objective:

To identify the chemical structure and formula of an unknown salt sample (FeCl₃) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be ferric chloride (FeCl₃), 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: Green

·       Physical State: Crystalline

·       Solubility: Soluble in cool water

Materials:

Chemicals:

·       Unknown salt sample (FeCl₃)

·       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, ruling out the presence of Pb²⁺ and Cu²⁺ ions.

·       Step 3: Heat the solution to expel H₂S gas. Add NH₄Cl and NH₄OH to make the solution basic. A brown jelly-like precipitate forms, suggesting the presence of Fe²⁺ or Al³⁺ ions.

·       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.    Fe²⁺/Fe³⁺ Confirmation Test:

·       Dissolve the brown jelly-like precipitate in water. The green solution formed indicates the presence of Fe³⁺.

·       Confirmation Test 1: Add potassium ferricyanide to the solution. A concentrated blue precipitate forms, confirming the presence of Fe²⁺/Fe³⁺ ions.

·       Confirmation Test 2: Add ammonium thiocyanate (NH₄SCN). A blood-red solution forms, further confirming the presence of Fe²⁺/Fe³⁺ ions.

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⁻ ions.

·       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 white precipitate confirms that 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: Fe³⁺ (ferric ion)

·       Anion Identified: Cl⁻ (chloride ion)

Thus, the unknown salt is identified as Ferric Chloride (FeCl₃).

Discussion Questions:

1.    What is ferric chloride?

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

3.    What color forms when potassium ferricyanide is added to ferric chloride solution?

4.    What happens when NH₄SCN is added to ferric chloride solution?