Lab Report Example: Solubility

Purpose of the Lab

The purpose of this lab was to investigate the solubility of various substances in different solvents with a view of making predictions based on the polarities of solutes and solvents. In addition, it was required to differentiate the miscible and immiscible pairs of liquids based on their characteristics (polarity).

Introduction

A substance is soluble in a solvent if it dissolves in that solvent and becomes part of the liquid. Solubility is controlled by the intermolecular forces between solute-solute, solvent-solvent, and solute-solvent molecules. Polar solutes are invariably soluble in polar solvents and nonpolar solutes soluble in nonpolar solvents. The polarity of organic molecules is determined by the presence of polar functional groups such as alcohols (-OH). Most organic molecules are relatively non-polar and are usually soluble in nonpolar solvents such as diethyl ether and hexanes but insoluble in polar solvents like water. Nevertheless, some organic molecules (with high ratio of polar groups to the nonpolar hydrocarbon chain) are more polar and therefore soluble in water. The presence of an acidic or basic group in a water-soluble compound can be detected by low or high pH, respectively, of the solution (Pavia, 2005).

Hypothesis

Compounds that are insoluble in water can become soluble if they are ionized (e.g. in acid or a base) because ionized forms much more polar.

Materials and Methods

Materials: Four dry test tubes, spatula, and micro spatula, water, Benzophenone, malonic acid, biphenyl, 1-octanol, 1-butanol , methyl alcohol, ethyl alcohol, diethyl ether, methylene chloride, hexane, methylene chloride, benzoic acid, and ethyl 4-aminobenzoate

Procedure: Approximately 40mg of benzophenone was placed into each of the four dry test tubes and the test tubes were labeled appropriately. 1 mL of water, 1 mL of methyl alcohol and 1 mL of hexane were added to the first, second, and third test tubes respectively. The fourth test tube was left as a control. Using the rounded end of the spatula each sample was stirred continuously for 60 seconds by twirling the spatula rapidly. The compound was said to be soluble in comparison with the control if it dissolved completely after 60 seconds, insoluble if none dissolves, and partially soluble if only some of it dissolved (at least 50%).

In the second experiment, 1 mL of the solvents (water or hexane) was added to a test tube then one of the alcohols (1-octanol, 1-butanol, or methyl alcohol) added dropwise. The solution was shaken continuously until 20 drops had been added. The reaction was observed carefully as each drop was added to the test tube and the results recorded in a table.

1 mL of water and 1 mL of ethyl ethanol was added to the same test tube. The test tube was shaken for 10 20 seconds to determine if the two liquids were miscible or immiscible. The process was repeated for each of the following pairs and the results noted: water/ diethyl ether, water/ methylene chloride, water/ hexane, and hexane/methylene chloride.

About 30 mg of benzoic acid was placed into each of the three test tubes and then labeled appropriately. 1 mL of water, 1 mL of 1.0 M NaOH, and 1.0 M HCl were added to the first, second, and third test tubes respectively. Each test tube was stirred with a micro spatula for 10-20 seconds. The compound was noted whether it is soluble or insoluble and the results recorded in a table. 6.0 M HCl was added dropwise to the test tube containing benzoic acid and 1.0 M NaOH until the solution became acidic (tested using a litmus paper). The acidic mixture was stirred for 10 20 seconds and the results noted in the table. The same procedure was repeated for ethyl 4-aminobenzoate but beginning with 1.0 M HCl and adding 6.0 M NaOH until the solution became basic. The results were noted in the table too.

Results

PART A

solid organic compounds SOLVENTS

water (highly polar) methyl alcohol (intermediate polarity ) hexane (nonpolar)

benzophenone insoluble partially soluble partially soluble

malonic acid soluble soluble insoluble

biphenyl insoluble insoluble soluble

PART B

Alcohols Water Hexane

1-octanol insoluble (the large, eight carbon alkyl group dominates the intermolecular forces found in the molecule) soluble

1-butanol partially soluble soluble

methyl alcohol soluble insoluble

PART C

solution miscibility

water and ethyl alcohol miscible

water and diethyl ether immiscible

water and methylene chloride immiscible

water and hexane immiscible

hexane and methylene chloride miscible

PART D

Compounds Water 1.0 M NaOH 1.0 M HCl

benzoic acid soluble soluble

Became insoluble upon adding 6.0 M HClinsoluble

ethyl 4-aminobenzoate insoluble insoluble soluble

became insoluble upon adding 6.0 M NaOHDiscussion

The findings of this laboratory experiment are consistent with the notion that polar solutes are soluble in polar solvents and nonpolar solutes soluble in nonpolar solvents. Benzophenone was found to be partially soluble partly in methyl alcohol and hexane but insoluble in water because the molecule is largely nonpolar, but possesses a polar carbonyl group that gives it partial polarity. Malonic acid is an easily ionizable polar molecule, which was found soluble in water and methyl alcohol, but insoluble in hexane. According to the results of Part A, biphenyl was found to be insoluble in water and methyl alcohol but soluble in hexane. This is because biphenyl is a nonpolar molecule and will only solubilize in nonpolar solvents (Pavia, 2005).

In part B, the results show that 1-octanol was soluble in hexane but insoluble in water due to its molecular formula. Water is considered a polar molecule because it has a dipole moment towards the oxygen. 1-octanol with water is insoluble because even though the OH group in 1-octanol is polar the long carbon chain (non-polar) over powers the polarity making it repel the polar water molecules. 1-butanol was also found partially soluble in hexane and water. The OH group in 1-butanol is polar. However, the carbon chain of the 1-butanol molecule, which is non-polar, allows it to repel the water, but not as much, making it partially soluble in both polar (water) and nonpolar solvents (hexane). Hexane is composed of carbons and hydrogens, which make it non-polar. As such, 1-butanol and 1-octanol, which are largely non-polar in nature due to their carbon chains, are soluble in hexane. Methyl alcohol was soluble in water and partially soluble in hexane. 1-methanol has only one carbon, which makes it an ineffective nonpolar group thereby making the molecule polar. This demonstrates that alcohols become more soluble in water (and less soluble in nonpolar solvents) as the alkyl portion of the alcohol reduces and vice versa (Pavia, 2005).

In part C, water and ethyl alcohol as well as hexane and methylene chloride were found to be miscible, while the following pairs were found to be immiscible water diethyl ether, water methylene chloride and water hexane.

In Part D, the investigation of the solubility of benzoic acid (an organic acid) and ethyl 4-aminobenzoate (an organic base) at different pH values found that benzoic acid was soluble in water and 1.0 M NaOH. However, the organic acid became insoluble in 6.0 M HCl and insoluble in 1.0 M HCl. On the other hand, the organic base (ethyl 4-aminobenzoate) was found to be insoluble in water and 1.0 M NaOH. However, ethyl 4-aminobenzoate became insoluble when 6.0 M NaOH was added to this solution, but soluble in 1.0 M HCl. This demonstrates that ionization of both the organic acid and the organic base makes them more soluble and non-ionization makes them less soluble.

Conclusions

This study investigated the solubility of various substances in different solvents with a view of making predictions based on the polarities of solutes and solvents. The study affirms that the presence of polar functional groups determines the polarity of compounds and that the higher the high ratio of polar groups to the nonpolar hydrocarbon chain, the more polar and therefore soluble in water. In addition, compounds that are insoluble in water can become soluble if they are ionized (e.g. in acid or a base) because ionized forms much more polar.

Reference

Laboratory Manual

Pavia, D. L. (2005). Introduction to organic laboratory techniques: a small scale approach. Cengage Learning.