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Synthesis of Organic Aspirin Essay

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The purpose of this experiment was to find out how a reaction undergoes for a globally known painkiller called aspirin, and to become familiar with achieving successful yields. Aspirin is synthesized from salicylic acid and acetic anhydride. Those two chemicals are mixed together along with sulfuric acid to form a crude solid. Filtration is used separate the impurities from the crude aspirin. To get purified aspirin, the precipitate was heated until all the excess had been removed. The end product is pure dry aspirin.

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Though there was too much room for error the yield percentage will almost never be a hundred percent.


In 1897 Felix Hoffman synthesized acetylsalicylic acid when he was treating is father with today’s so called aspirin due to his suffering of arthritis. It was known that aspirin had some side effects that were caused by the level of acid in the aspirin itself, which made the stomach irate when ingested. Hoffman reduced the acid by mixing the aspirin with sodium salicylate, a extract found in the willow tree bark [1], which paved the way to today’s most popular and inexpensive drug [2].

Aspirin is the common name given to acetylsalicylic acid, sometimes abbreviated as ASA. ASA is considered to be an organic acid since it is composed of Carbon, Hydrogen, and Oxygen which also contains the carboxyl functional group-COOH. Pure ASA can be synthesized by treating salicylic acid with acetic anhydride in the presence of a trace of either sulfuric (in our case) or phosphoric acids. The complete reaction of ASA is shown in Figure 1.1 on page 2. Figure 1 shows the reaction for the synthesis of aspirin.

Figure 1: Salicylic Acid + Acetic Anhydride ( ACETYLSALICYLIC ACID + Acetic Acid


There were plenty of chemicals that were used in this experiment. Table 1 shows all chemicals used in the experiment and the source of them. For more depth look at the chemicals used please refer to Table 1.2 on page 2.

The equipment that was used was very much basic except the odd few. Table 1.1 provides a list of all the equipment used. Common lab room equipment is not listed.



1 Graduated Cylinder

1 Scoopula

1 Analytical Balance

1 Hot Plate

1 Eye Dropper

Adding chemicals.

Scraping solids of filter paper.

Recording Weights.

Drying the solids.

Adding distilled water.

Table 1.1: Equipment used and purpose.

Experiment starts off by taking salicylic acid and mixing it with the acetic anhydride. To this adding five drops of sulfuric acid and some distilled water gave a forming solid after the solution cooled. The crude aspirin had formed. Since pure aspirin was what we are after, filtration needed to take place. Filtration is a treatment in which you remove the solid from the water [3]. End of the experiment, heating the precipitate allowed it to dry and form an end “powder” substance of purified aspirin. Figure 2 shows the process of filtration. For more precise and descriptive procedure, follow the procedure listed in the University of Winnipeg, Department of Chemistry, Laboratory Manual (2007) for CHEM1111/3, page 4-3.


To determine the limiting reaction we need to take a closer look at the equation. First we need to see if it is balanced. As shown in Figure 1 we see that the equation is balanced on both the reactant and product side. Since the limiting reagent is the reactant that is completely consumed in the reaction we need to calculate moles. The equation for moles and calculations is listed in the Appendix on this report. Since the salicylic acid is being completely used up in the reaction, this is the limiting reagent.

The theoretical yield is the actual mass of the ASA in the reaction. For calculations, the moles of salicylic acid (limiting reagent) need to be multiplied to the molar mass of ASA. This gives the theoretical value. In contrast the percent yield explains how far or how close the theoretical value is to the percent error. To calculate percent yield, the mass of the experimented ASA needs to be divided by the theoretical value. This gives a decimal number, in which needs to be multiplied by a hundred to get the yield percentage [4].

The reason why we use cold water in the experiment rather than warm water is to how well we want the precipitate to form. In recrystalization if warm water was used then the precipitate might have not formed that well. Since the product must remain a solid and not dissolve cold water was used to basically keep the precipitate [5].

In recrystalization the reason that the same filter should be used twice is because when scraping off the aspirin the error is that you never get all of it off the old filter paper. When this happens your mass decreases and inaccurate results will show. When using the same filter paper you are putting the new pure aspirin back on the filter paper where it came from, resulting in no loss of aspirin and the mass remains constant.

Sources of determinate error in this experiment were very easy to make as human errors. There was accidental error when more than five drops of sulfuric acid was added to the mixture of salicylic acid and acetic anhydride because the turning knob on the burette was faulty. This could have affected my results since sulfuric acid was the catalyst in this reaction causing inaccurate results.

When it was time to weigh the filter paper after the dry aspirin was scraped off, the error could be that the water which was likely to still be in the filter paper this could have added more weight to the filter paper alone. This is error because it gives me incorrect weight for pure ASA product leading to an inflated yield percentage. However theoretical percentage will remain unchanged because it is based on the amount of possible product [6]. This could have been solved by letting the filter paper sit on the hot plate for a longer period of time. There were minor human errors like inaccurate measurements due to negligence. The seemingly obvious remedy would be to pay more attention to the experiment and keep a close on eye on it.


Acetylsalicylic acid (ASA), or more commonly known as aspirin is synthesized from the reaction of salicylic acid mixing with acetic anhydride. The end product is aspirin, but in the crude form. Crude is the form of aspirin that contains impure acids, like unreacted sulfuric acid. These impurities need to be separated from the crude aspirin to achieve the pure dry form. This was done through recrystalization.

Recrystalization is the treatment process of taking the crude aspirin which is contaminated with the impure sulfuric and salicylic acid and separating them from the aspirin. The end product is pure dry aspirin in our case. In conclusion I think that the experiment was poorly done with too many errors. The cause of this could be that I did not pay enough attention or most likely ran out of time at the end of the experiment.


[1] www.infoplease.com. Pearson Education. Nov 14 2007.

[2] Olmsted III, John. “Synthesis of Aspirin”. Oct 1998.

Journal of Chemical Education

[3] www.saskschools.ca. Nov 13 2007.

[4] www.fomo.com. Fomo Products, Inc. Nov 13 2007.

[5] Chemistry.about.com. Anne Marie Helmenstine, PH. D. Nov 13 2007.


The mass of acetic anhydride is necessary to calculate the limiting reagent which leads to out percent error. To find mass, simply take the given density and multiply it by the given value.

To find out the limiting reagent, the moles of the reactants salicylic acid and acetic anhydride need to be known. The molecular mass is given for each reactant. Simply take the mass of each reactant and divide it by the given molecular weight.

The same equation can be used to find out the mollies of the acetic anhydride.

The limiting reagent is the reactant that has the least moles; in this case the salicylic acid is smaller than the acetic anhydride. To figure out the limiting reagent look at which reactant gets completely used up in the reaction. In this reaction it is the salicylic acid. From this the mass of the ASA needs to be calculated. Since the reaction is a one-to-one ratio this means the same moles of the reactant agent can be applied to the ASA to find mass.

Now that we have the limiting reagent and the mass of the ASA we can find out the percent yield of ASA. It is predicted that the percentage will not be anywhere close to 100% due to possible errors during the experiment. To find the yield percentage we take the mass of the ASA and divide it by the experiment calculated mass of ASA. This will give us a decimal, in which we can calculate the percentage.

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