Experiment 159

Objective

To identify the presence of acetic acid in an unknown organic sample.

Introduction

This experiment involves the analysis of physical characteristics, solubility testing, and functional group tests to systematically identify the unknown compound. The melting point comparison further confirms the compound's identity.

Physical Characteristics of Sample:

·       Color: Colorless

·       State: Liquid

·       Odor: Odorless

·       Melting Point: 16.7°C

Solubility Test Results

Materials

Chemicals

·       Supplied sample (presumed acetic acid, 99.5%)

·       Distilled water

·       5% HCl, 5% NaOH, 5% NaHCO₃

·       Ferrous sulfate, Nitric acid

·       Silver nitrate, Ferric chloride

·       10% NaNO₂, 2% bromine solution

Apparatus

·       Test tubes and holder

·       Bunsen burner

·       Thermometer

·       500 mL flask, test tube stand

·       Pipette

·       Fusion tube

·       Mortar and pestle

Preparation of Reagents

·       Prepare solutions of 5% HCl, 5% NaOH, 5% NaHCO₃, and 5% ferrous sulfate in a 500 mL flask.

·       Prepare a 10% NaNO₂ solution and a 20% NaOH solution for specific tests.

Procedure

1.    Elementary Analysis (Stock Solution Preparation)

Heat a piece of sodium in a fusion tube until molten.Add a small amount of the sample, heat to a dull red, then drop the tube into a mortar with 3-5 mL of distilled water.Crush the tube, filter the solution, and use it for further testing.Test for Nitrogen: Add 1-2 mL of the stock solution to freshly prepared ferrous sulfate, boil, cool, and add a few drops of H₂SO₄.Observation: No Prussian blue indicates nitrogen is absent.Test for Halogens: Acidify 1 mL of boiling solution with nitric acid, boil, cool, and add silver nitrate.Observation: Absence of white, pale yellow, or yellow precipitate indicates no halogens (Cl, Br, I).Test for Sulfur: Acidify 1 mL of solution with acetic acid and add lead acetate.Observation: No black precipitate, indicating sulfur is absent.

2. Functional Group Tests

Test for Unsaturation: Dissolve 0.2 g of the sample in CCl₄, add bromine solution.Observation: No discharge of bromine color, indicating unsaturation is absent.Nitro Group (-NO₃): Boil with tin and HCl, cool, dilute with water, and add NaNO₂.Observation: No orange-red precipitate, indicating -NO₃ group is absent.Carbonyl Group (=C=O): Dissolve in rectified spirit, add 2,4-DNPH.Observation: No orange-red precipitate, indicating carbonyl group is absent.Carboxylic Acid (-COOH): Add sample to NaHCO₃ solution.Observation: Effervescence of CO₂, turning limewater milky, confirms -COOH.Phenol Group (Ar-OH): Dissolve in rectified spirit, add FeCl₃.Observation: No intense coloration, indicating phenol group is absent.Aromatic Hydrocarbon: Dissolve in absolute alcohol, add picric acid solution.Observation: No orange-red needle-shaped crystals, indicating absence of aromatic hydrocarbons.

Observation

Identified Functional Groups:

Elements: None (N, X, S absent)Functional Groups: Carboxylic acid (-COOH)

Based on the melting point of 16.7°C and the presence of these functional groups, the sample is identified as acetic acid (glacial acetic acid).

Questions

1.    What is a fatty acid?

2.    How is acetic acid identified?

3.    What reagent is used in the -COOH group test?

4.    Why is acetic acid classified as a fatty acid?

5.    What color does acetic acid exhibit at 16.7°C?