Experiment 127

Objective:

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

Introduction:

The unknown salt, suspected to be cobalt nitrate (Co(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: Pink

·        Physical State: Crystalline

·        Solubility: Soluble in cool water

Materials:

Chemicals:

·       Unknown salt sample (Co(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, 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. No precipitation suggests the absence of Fe²⁺ and Al³⁺ ions.

·       Step 4: Flow H₂S through the basic solution again. The formation of black 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.    Co²⁺ Confirmation Test:

·       Dissolve the black precipitate in water. Add potassium chlorate and HCl, then heat the solution to obtain a blue solution. Divide the solution into three portions:

·       Portion 1: Add NH₄OH and NH₄Cl, followed by di-methyl glyoxime. The absence of precipitation confirms that Ni²⁺ is not present.

·       Portion 2: Add NH₄SCN and amyl alcohol. The alcohol layer turns red, confirming the presence of Co²⁺.

·       Portion 3: Add dilute nitric acid and potassium nitrate. Co²⁺ is present.

4.    Acidic Group (Anion) Tests:

·       Step 1: Add nitric acid followed by silver nitrate to the sample solution. No white 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. No precipitation confirms that SO₄²⁻ is absent.

·       Step 4: Ring Test: Add concentrated sulfuric acid to the solution. Cool it and carefully layer ferrous sulfate on top. The formation of a brown ring between the layers confirms the presence of NO₃⁻ ions.

Observations:

·        Cation Identified: Co²⁺ (cobalt ion)

·        Anion Identified: NO₃⁻ (nitrate ion)

Thus, the unknown salt is identified as Cobalt Nitrate (Co(NO₃)₂).

Discussion Questions:

1.    What is cobalt nitrate?

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

3.    What happens when NH₄SCN and amyl alcohol are added to cobalt salt?