Aspirin is chemically synthesized by acetylation with salicylic acid and acetic anhydride, explains the Aspirin Foundation. The resulting compound, known as acetylsalicylic acid, has a chemical formula of C9H8O4. Aspirin in this form is 100 percent acetylsalicyclic acid.
Manufacturers of aspirin tablets use fillers and binders to create the tablet. For example, Bayer aspirin contains corn starch, hypromellose, powdered cellulose and triacetin, as of 2007, and it may also include carnuba wax, according to RxList. Triacetin facilitates absorption of the active ingredient, acetylsalicylic acid, and serves as a filler to increase the volume of the tablet, according to Wikipedia. Uncoated aspirin tablets contain around 90 percent acetylsalicylic acid, according to Wikipedia.