CHM 1025C/CHM 1032C Experiment 14: Synthesis of an Acid


Purpose: To synthesize an acid to observe the process of purification

via re-crystallization



1. The student will gain some knowledge of synthetic methods.

2. The student will learn the techniques of crystallization and re-crystallization to purify a synthesized substance.

3. The student will re-enforce the technique of determining melting points on the unknown acid.

4. The student will learn to collect a solid product on a Buchner funnel by vacuum filtration.


Equipment and Chemicals:

1 thermometer, 260 C

1 250 ml Erlenmeyer flask

1 125 Erlenmeyer flask

Buchner funnel

Filter flask

Gooch ring

Filter paper

Capillary tubes

Meltemp Apparatus

Stirring rod

Ring stand

Iron ring


Phosphoric acid

Acetic anhydride

Salicylic acid

1% Ferric chloride solution

Commercial aspirin















In this experiment you will synthesize acetylsalicylic acid. The procedures for synthesizing the acid appear in the procedure section below.


Acetylsalicylic acid is also called aspirin. It is an anti pyretic substance (has the ability to lower high body temperatures to normal) and an analgesic substance (able to relieve minor pain). It is a moderately effective agent in the treatment of rheumatoid arthritis. Thirty million pounds usually in 5 grain tablets are used annually in the United States alone. Most of the aspirin is produced by the reaction you will use. Salicylic acid is reacted acetic anhydride to form acetylsalicylic acid. It is important to purify this compound because of contaminating acid.




The synthesis of aspirin is classified as an esterification reaction. Salicylic acid is treated with acetic anhydride, an acid derivative, causing a chemical reaction that turns salicylic acid's phenol group into an acetyl group, (R-OH → R-OCOCH3). This process yields aspirin and acetic acid, which is considered a byproduct of this reaction. Small amounts of sulfuric acid (and occasionally phosphoric acid) are almost always used as a catalyst.


Procedure: Acetylsalicylic Acid (Aspirin)


(1) Place 2.5-3.0 grams of salicylic acid in a 125 ml Erlenmeyer flask,

(2) Add 8 ml of acetic anhydride and 15 drops of 85% phosphoric acid (H3PO4)


Caution: Acetic anhydride has a peppery smell and is an eye irritant. If you spill

acetic anhydride on the desk, do not wash it with water.


(3) Stir the mixture well.

(4) Heat the flask in a water bath for 5 minutes.

(5) Remove the flask from the bath and, while still hot, cautiously add 3 ml of water in one portion.


Caution: The solution may boil from the heat of decomposition of the excess

acetic anhydride; handle the flask carefully!


(6) After the decomposition is complete, add 40 ml of distilled water and stir the solution until crystals begin to form.

(7) Cool the mixture in an ice bath to complete the crystallization process.

(8) Collect the acid by vacuum filtration.

(9)Wash with cold water and allow the product to dry on the Buchner funnel.

(10)Take a small sample of the aspirin and place it in a test tube and 1 ml of water.

(11) Add 1 drop of 1% Ferric chloride solution to test for the phenolic hydroxyl group.

Note the color and repeat the test on both commercial aspirin and pure salicylic acid. If your aspirin is dark purple in color from this test, wash the aspirin with more cold water until the test is less purple than the commercial aspirin.

(12) Remove a sample a dry it and determine the melting point of the aspirin.

(13) Weigh the dry sample next week .

(14) Save the sample for you to take home and remember this class by keeping it in your medicine cabinet


Calculations for your Lab Notebook:

1. When the sample is dry (next week), weigh the sample.

2. Write the equation for the reaction

3. Calculate the theoretical yield of the acid you synthesized from the beginning amount of Salicylic acid used.

4. Using the actual mass of the synthesized acid and the theoretical yield, calculate the percent yield for this experiment.