Theory
Salicylic acid + Acetic anhydride → Aspirin + Acetic acid
(C7H6O3 + C4H6O3 → C9H8O4 + CH3COOH)
Catalyst: conc. H2SO4 / H3PO4
Salicylic acid: MW = 138 g/mol; mass taken = 2.00 g
Aspirin: MW = 180 g/mol; theoretical mass = 2.61 g

Steps

1. Weigh salicylic acid (2.00 g)
2. Add acetic anhydride (3 mL)
3. Add catalyst (H2SO4)
4. Heat on water bath (15 min)
5. Cool and add cold water
6. Filter and wash crystals
7. Dry, weigh, and test purity

Yield

Complete preparation to calculate.

Safety Check

  • Use goggles, gloves, and fume-hood conditions for acetic anhydride and concentrated acid.
  • Add acid catalyst dropwise; the mixture can warm quickly.
  • Use a water bath only. Avoid naked flames with organic reagents.
  • Collect acidic filtrate and washings as organic/acid waste.
Round-bottom flask (empty)

Mechanism Focus

The phenolic -OH group of salicylic acid is acetylated. Acid catalyst activates acetic anhydride, aspirin forms, and acetic acid is the by-product.

Purity Check

A sharp melting point near 136°C suggests comparatively pure aspirin. A low or broad range suggests moisture, unreacted salicylic acid, or trapped acetic acid.

Lab Notebook
Observations will appear here...

Predict Before Heating

Which reagent limits the maximum mass of aspirin, and why should cold water help crystals appear?

Current Interpretation

Begin by weighing salicylic acid accurately. This mass controls the theoretical yield used at the end.