Experiment 118

Objective

To analyze and identify the basic and acidic components in an unknown salt sample, hypothesized as sodium sulfate (Na₂SO₄).

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: Colorless

●      Physical State: Crystal

●      Solubility: Soluble in cold water

Materials

Chemicals

●      Sample (presumed as Na₂SO₄)

●      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, indicating absence of Fe²⁺ and Al³⁺ ions

4.    Secondary Hydrogen Sulfide Test

●      Introduce H₂S again in the basic solution.

●      Observation: No precipitate; indicates absence of Ni²⁺, Co²⁺, and Zn²⁺ 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. Sample presence suggests possible Na⁺ or K⁺ ions.

7.    Confirmatory Tests for Sodium Ion (Na⁺)

●      Sodium: Add potassium pyroantimonate solution. White precipitate confirms Na⁺ presence.

●      Potassium Test: Add concentrated tartaric acid and alcohol. No precipitate; K⁺ is absent.

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): Sodium ion (Na⁺) is present.

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

Conclusion: The sample is identified as sodium sulfate (Na₂SO₄).

Questions

1.    What is sodium sulfate?

2.    What happens when potassium pyroantimonate is added to a sodium sulfate solution?

3.    What happens when barium chloride is added to a sodium sulfate solution?