Course: Organic Chemistry 1
Section: 2423 SP1
Term: Fall 2018
Instructor: Dr. Ludivina Avila
By: Idalia Meza
Experiment 2: Recrystallizing Impure Benzoic Acid
The purpose of the lab is to recrystallize impure benzoic acid, as well as also obtaining the infrared spectroscopy, melting point, and calculate the percent recovery. First, students must have all materials needed. Then, set up an apparatus needed for the infrared spectroscopy and the determination of the melting point. The results of the recrystallization should be for crystals to reform in a pure colorless state. In this project, students learned how to recrystallize a sample of impure benzoic acid.
In this experiment students will learn to recrystallize a sample of impure benzoic acid and determine the percent recovery of the sample, analyze the purity using melting point, and infrared spectroscopy.
This experiment will require an extra amount of time, approximately 10-15 minutes, for the crystals to form while they are being dried by the vacuum.

For this lab some materials include contaminated benzoic acid (about 2.0g), two 100 mL beakers, two 250 mL beakers, a 125 mL Erlenmeyer flask, a hot plate which will be used to heat the solution in. Disposable pipets, glass stirring rods, set up a vacuum filtration apparatus to dry the pure Benzoic Acid into crystals, coffee filters to filter the hot solution, distilled water (100 mL) for rinsing of the Benzoic Acid. Ice, a watch glass, powder funnel, mortar and pestle to grind the newly formed crystals, melting point capillaries to determine the melting point of Benzoic Acid, a melting point apparatus, and an infrared spectrophotometer to determine how much of the impure benzoic acid was recovered.
Begin this lab by wearing safety attire. Obtain 2.0g of the impure benzoic acid. Transfer the amount into a 125mL Erlenmeyer flask, and add 45mL of distilled water, and boil. If the impure benzoic acid has a color, add active carbon to help the benzoic acid go to a clear color. While the solid dissolves set up a vacuum filtration. Once benzoic acid is dissolved, transfer it to the vacuum apparatus and dry the solution into crystals. Then crush the crystals into a fine powder, and place a very small amount inside a capillary tube and get the melting point of it, do this twice. Finally get an infrared spectrum reading to determine how much pure benzoic acid was recovered. If it ranges from 900-1,000, then a good amount of benzoic acid was recovered.
Some observations that were made when recrystallizing the benzoic acid were that benzoic acid as an impure weighed more (2.007g) than the recrystallized pure benzoic acid (0.986g). As this observation was made, it could be considered that the impurities in benzoic acid cause it to weigh more, than the pure form of it.

Data collected
Mass of impure Benzoic Acid……………..2.007g
Mass of filter paper…………………………….0.2161g
Total mass of filter paper/Benzoic Acid……1.202g
Mass of recrystallized Benzoic Acid……….0.986g
% Recovery………………………………..49%
Melting point
Trial 1……………………………..122C
Trial 2……………………………..123C
Literature melting point……………………122C
Reference for literature melting point:
% Error (melting point)…………………………..0%
% Recovery
% Recovery = (mass of pure substance (g)/mass of impure substance (g))x100%
% recovery = (0.986g/2.007g)x100% = 49.13 = 49%
% Error
% error = {theoretical value-observed value/theoretical value}x100%
% error = {122C-122C/122C}x100% = 0%
The objective of this experiment was to recrystallize impure Benzoic Acid, obtain the percent recovery of the pure Benzoic Acid along with its melting point, and infrared spectroscopy. After placing a sample of the already pure Benzoic Acid in the spectrophotometer, the machine read a value of 968 out of 1,000 and confirmed it as Benzoic Acid. Also the literature value of the melting point of Benzoic Acid is 122C and in the experiment (on both trials for the melting point) the temperature for melting obtained was 122C for trial one and 123C for trial two. Which means that melting point when compared to the literature melting point was correct.

CHEM 2423 Laboratory Manual, STC Chemistry Department, 2018
Pre- Lab Questions:
Crystallization forms crystals; solids whereas precipitation forms liquid; drops.

Increasing temperature.

– dissolve the solute in solvent
– perform a filtration
– obtain crystals of solute
– collect crystals by vacuum filtration
– dry resulting crystals
If your impure Benzoic Acid has a color carbon will be used to remove colored impurities.

The purpose of measuring the melting point and recording an infrared spectrum of the sample is to determine the melting point (temperature) of sample and to see how pure it is after recrystallization.

It is important to use ice cold water to rinse off purified crystals because impurities might be trapped, and therefore ice cold water is used to wash the Benzoic Acid crystals in order to dissolve them and their impurities.
0.02g per 100 mL of water.

Percent recovery is the amount of pure compound with respect to impure compound obtained from chemical synthesis and percent yield is the amount of a compound obtained from a chemical synthesis reaction with respect to the theoretically expected amount.

Post- Lab Questions:
1. The student should obtained “active carbon” which will help the impure sample go from a greyish color to clear.

2. The same amount of Benzoic Acid would be left.

3. Instead of starting off the experiment with boiling the benzoic acid in water at a hot temperature, it can be dissolved in methanol at a hot temperature, and when crystals must be formed use water.