Theory — The Aspirin Reaction

Aspirin is one of the oldest and most widely used medicines. It is prepared by an esterification: the hydroxyl (OH) group on salicylic acid reacts with acetic anhydride, replacing the hydrogen of that group with an acetyl group and forming the ester acetylsalicylic acid. Acetic acid is released as a by-product.

The balanced reactionsalicylic acid + acetic anhydride → acetylsalicylic acid (aspirin) + acetic acid
C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + C₂H₄O₂
One mole of salicylic acid gives one mole of aspirin (a 1 to 1 ratio)

The role of each reagent

Each substance in the flask has a job:

SubstanceMolar mass (g/mol)Role
Salicylic acid (C₇H₆O₃)138.12The limiting reagent; it provides the aromatic ring and becomes aspirin.
Acetic anhydride (C₄H₆O₃)102.09The acetylating agent, added in excess so all the salicylic acid reacts.
Phosphoric acid (H₃PO₄)—A few drops act as a catalyst, speeding the reaction without being used up.
Water—Added at the end to destroy the excess anhydride and start crystallization.
Aspirin (C₉H₈O₄)180.16The product; a white crystalline solid that melts near 135 degrees Celsius.

Theoretical yield, percent yield, and purity

Because the reaction is 1 to 1, the moles of aspirin you can make equal the moles of salicylic acid you start with. Converting those moles to grams gives the theoretical yield, the most product possible. The mass you actually recover, divided by the theoretical yield, gives the percent yield. Finally, a pure substance melts sharply at its characteristic temperature, so a melting point at or just below 135 degrees Celsius with a narrow range tells you the aspirin is pure; a low, broad range signals impurity.

Key equationsmoles of salicylic acid = mass ÷ 138.12
theoretical mass of aspirin = moles × 180.16
percent yield = (actual mass ÷ theoretical mass) × 100
The percent yield is almost always below 100% because of losses in transfer, filtration, and washing

Limiting reagent

Salicylic acid limits the product because the acetic anhydride is in excess; the moles of aspirin follow from it.

Yield reflects practice

Spills, incomplete reaction, and product left in solution lower the percent yield below the theoretical value.

Melting point tests purity

A sharp melt near 135 degrees Celsius means pure aspirin; a depressed, broad range means impurities remain.

Apparatus

The instruments and glassware you set up on the bench to synthesize, isolate, and test the aspirin.

12.4736 g TARE
Analytical balance
Weighs the salicylic acid before the reaction and the dry aspirin afterward.
Erlenmeyer flask
Holds the salicylic acid, acetic anhydride, and catalyst during the reaction.
Graduated cylinder
Measures out the acetic anhydride and the water added at the end.
Water bath
Warms the flask gently to about 80 degrees Celsius to drive the reaction.
100 0
Thermometer
Checks the bath temperature so the mixture is heated but not boiled dry.
Ice bath
Cools the mixture so the aspirin crystallizes out of solution.
to vacuum
Vacuum filtration
Collects the crystals on a Buchner funnel and pulls the liquid away under vacuum.
135.4 C
Melting point apparatus
Heats a sample of the dried product to find its melting range and judge purity.

Instructions — Running the Virtual Experiment

This is a predict, reveal, and compare lab. You record your masses, work out each result by hand, enter it, and only then does the simulation reveal the value so you can compare.

Synthesis (read first)
1
Weigh the salicylic acid into a dry Erlenmeyer flask. Add the excess acetic anhydride and a few drops of phosphoric acid catalyst.
2
Warm the flask in the water bath at about 80 degrees Celsius for fifteen minutes, swirling. Then add cold water to destroy the unreacted anhydride.
3
Cool the flask in an ice bath until crystals form, collect them by vacuum filtration, wash with cold water, and let the product dry before weighing.
Part A — Theoretical Yield (Theoretical Yield tab)
1
Choose your starting mass of salicylic acid. Calculate the moles of salicylic acid (mass divided by 138.12) and then the theoretical mass of aspirin (moles times 180.16), enter both, and click Check.
Part B — Percent Yield (Percent Yield tab)
1
The balance shows the mass of dry aspirin you recovered. Calculate the percent yield (actual mass divided by theoretical mass, times 100), enter it, and click Check.
Part C — Purity by Melting Point (Purity tab)
1
Select a product sample and read its melting range from the apparatus. Predict whether the sample is pure by comparing with the literature value of 135 degrees Celsius, then click Check.
For your reportInclude the balanced reaction, the role of each reagent, your recorded masses, your theoretical and percent yield calculations, the melting range and what it tells you about purity, and screenshots.

Simulation — The Aspirin Bench

Aspirin Synthesis Virtual LabRecord, calculate, then reveal and compare
QuantityYour valueActual
Choose a starting mass, calculate, and check.

Starting material

Limiting reagent: salicylic acid
Moles of salicylic acid— hidden
Theoretical mass of aspirin— hidden
From your synthesis (starting mass 2.00 g)
Theoretical mass of aspirin2.609 g
Dry aspirin recovered (balance)2.05 g

Percent yield

Set the same starting mass you used in Part A, read the recovered mass, then calculate the percent yield.

Percent yield— hidden
Melting point apparatus
Observed melting range— select a sample
Literature value (pure aspirin)135 °C

Purity check

Assessment— hidden

Team Questions

Question 1. What is the chemical name of aspirin? (two words)
Question 2. Which reagent is the limiting reagent in the synthesis? (two words)
Question 3. Which reagent supplies the acetyl group and is added in excess? (two words)
Question 4. What is the mole ratio of salicylic acid to aspirin in the balanced equation? (write as a ratio like 1:1)
Question 5. The molar mass of aspirin is about 180 g/mol. How many grams of aspirin could 0.0100 mol form? (to one decimal place)
Question 6. Pure aspirin melts near what temperature in degrees Celsius? (whole number)
Question 7 — Challenge. If the theoretical yield is 2.61 g and you recover 2.05 g, what is the percent yield? (whole number)

Example Lab Report

A worked example showing the expected format and the record, calculate, and compare workflow.

Synthesis of Aspirin

Chemistry | Section: [Your Section] | Date: [Date]

Lab Members: [Names of all members present]

Objective

To synthesize acetylsalicylic acid from salicylic acid and acetic anhydride, to determine the theoretical and percent yield, and to assess the purity of the product from its melting point.

Results (worked example)

QuantityValue
Mass of salicylic acid2.00 g
Moles of salicylic acid (2.00 / 138.12)0.01448 mol
Theoretical mass of aspirin (0.01448 × 180.16)2.609 g
Dry aspirin recovered2.05 g
Percent yield (2.05 / 2.609 × 100)78.6 %
Observed melting range134 to 135 °C

The 1 to 1 mole ratio gives a theoretical yield of 2.61 g from 2.00 g of salicylic acid. Recovering 2.05 g is a percent yield of about 79 percent, with the loss explained by crystals left in solution and on the glassware. The melting range of 134 to 135 degrees Celsius, narrow and at the literature value, indicates the product is pure aspirin.

Discussion and Conclusion

Salicylic acid was the limiting reagent, with acetic anhydride in excess and phosphoric acid as catalyst. The percent yield of about 79 percent is typical for this preparation, and the sharp melting point near 135 degrees Celsius confirms a pure product. A lower, broader melting range would have indicated the need for recrystallization.

Practice Questions

Question 1
Starting from 2.50 g of salicylic acid, what is the theoretical yield of aspirin?
Hint: 2.50 / 138.12 = 0.01810 mol; 0.01810 × 180.16 = 3.26 g.
Question 2
Why is acetic anhydride added in excess rather than in an exact 1 to 1 amount?
Hint: the excess drives the reaction to completion so that all of the limiting salicylic acid is converted to aspirin.
Question 3
A student recovers 3.05 g of aspirin from a theoretical yield of 3.91 g. What is the percent yield?
Hint: 3.05 / 3.91 × 100 = about 78 percent.
Question 4
A product melts over the range 120 to 130 degrees Celsius. Is it pure aspirin? Explain.
Hint: no; the range is low and broad compared with the sharp literature value of 135 degrees Celsius, so impurities remain and it should be recrystallized.
Question 5 — Challenge
Explain two reasons a percent yield is usually less than 100 percent in this synthesis.
Hint: product stays dissolved in the mother liquor and is lost, and some is lost in transfer, on the filter, and during washing.