Experiment 119 

Objective

To analyze and identify the basic and acidic components in an unknown salt sample, hypothesized as magnesium carbonate (MgCO₃).

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: Non-crystalline

●      Solubility: Soluble in HCl

Materials

Chemicals

●      Sample (presumed as MgCO₃)

●      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 dilute HCl.

●      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 black 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; suggests Ni²⁺, Co²⁺, and Zn²⁺ ions are absent.

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. White precipitate forms, indicating Mg²⁺ is present.

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

7.    Confirmatory Test for Magnesium Ion (Mg²⁺)

●      Test: Add NH₄OH and Na₂HPO₄ to form a white precipitate, confirming Mg²⁺ presence.

Analysis of Acidic Group (Anion) in Salt

1.    Preliminary Test 

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

●      Observation: No white 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; no precipitate forms, confirming SO₄²⁻ absence.

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

●      Carbonate Test: Add BaCl₂. White precipitate forms, which dissolves in HCl with CO₂ gas release, confirming CO₃²⁻ presence.

Observations

●       Cation (Basic Group): Magnesium ion (Mg²⁺) is present.

●       Anion (Acidic Group): Carbonate ion (CO₃²⁻) is present.

Conclusion: The sample is identified as magnesium carbonate (MgCO₃).

Questions

1.    What is magnesium carbonate?

2.    What happens when NH₄OH and Na₂HPO₄ are added to magnesium salt?

3.    What color forms when NH₄OH and Na₂HPO₄ are added to magnesium salt?

4.    What happens when HCl is added to magnesium carbonate?