Experiment 115

Objective

To analyze and identify the basic and acidic components in an unknown salt sample, hypothesized as cobalt sulfate (CoSO₄).

Introduction

The salt is identified through tests for basic and acidic groups, enabling determination of its chemical structure and formula.

Physical Properties of Sample

●      Color: Pink-red

●      Physical State: Crystal

●      Solubility: Soluble in cold water

Materials

Chemicals

●       Sample (presumed as CoSO₄)

●       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 test tube holder

●      Bunsen burner

●      Thermometer

●      500 mL flask

●      Test tube stand

●      Pipette

Reagents Preparation

●      Sample Solution: Dissolve 1.275 g of the supplied sample in 50 mL of distilled water.

●      5% HCl Solution: Prepared in a beaker.

●      Ammonium Hydroxide Solution: Prepared in a beaker.

●      Ferrous Sulfate Solution: 5% solution prepared in a 250 mL flask.

Procedure

Analysis of Basic Group (Cation) in Salt

1.    Preliminary Test

●      Place the sample in a test tube and add dilute HCl.

●      Observation: No precipitate; indicates absence of Pb²⁺ ions.

2.    Hydrogen Sulfide Test

●      Heat the sample and introduce H₂S gas.

●      Observation: No precipitate; confirms absence of Pb²⁺ and Cu²⁺ ions.

3.    Ammonium Hydroxide and Ammonium Chloride Test

●      Add NH₄Cl and NH₄OH to make a basic solution.

●      Observation: No precipitate, suggesting absence of Fe²⁺ and Al³⁺ ions.

4.    Secondary Hydrogen Sulfide Test

●      Introduce H₂S again in the basic solution.

●      Observation: Black precipitate forms, indicating possible presence of Co²⁺ ions.

5.    Carbonate Test

●      Heat the solution to remove H₂S, then add NH₄OH, NH₄Cl, and (NH₄)₂CO₃.

●      Observation: No precipitate, confirming absence of Ba²⁺ and Ca²⁺ ions.

6.    Division of Solution

●      Portion 1: Heat, add NH₄OH and Na₂HPO₄. No precipitate, indicating Mg²⁺ is absent.

●      Portion 2: Vaporize and dry. No sample presence, suggesting Na⁺ and K⁺ are absent.

7.    Confirmatory Tests for Cobalt Ion (Co²⁺)

●      First Test: Add NH₄Cl, NH₄OH, and dimethyl glyoxime to a portion. No precipitate; Ni²⁺ is absent.

●      Second Test: Add ammonium thiocyanate (NH₄SCN) and amyl alcohol. Red color in the alcohol layer confirms Co²⁺.

●      Third Test: Add dilute nitric acid and potassium nitrate. Color change confirms Co²⁺ presence.

Analysis of Acidic Group (Anion) in Salt

1.    Preliminary Test

●      Add nitric acid and silver nitrate to the sample solution.

●      Observation: No precipitate, suggesting absence of Cl⁻, Br⁻, or I⁻ ions.

2.    Additional Tests

●      Sulfide Test: Add sodium nitroprusside; no violet color observed, indicating absence of S²⁻.

●      Sulfate Test: Add dilute HCl and barium chloride; white precipitate confirms SO₄²⁻ presence.

●      Nitrate Ring Test: Add concentrated H₂SO₄, then ferrous sulfate to form a layer. Absence of a brown ring confirms NO₃⁻ is not present

Observations

●      Cation (Basic Group): Cobalt ion (Co²⁺) is present.

●      Anion (Acidic Group): Sulfate ion (SO₄²⁻) is present.

Conclusion: The sample is identified as cobalt sulfate (CoSO₄).

Questions

1.    What is cobalt sulfate?

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

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

4.    What happens when barium chloride is added to cobalt sulfate solution?