The optimal molar ratio of acetic anhydride to salicylic acid for the synthesis of acetylsalicylic acid aspirin is 1:1. This is because the reaction between acetic anhydride and salicylic acid is a 1:1 stoichiometric reaction, meaning that one mole of acetic anhydride reacts with one mole of salicylic acid to produce one mole of acetylsalicylic acid and one mole of acetic acid as a byproduct.However, in practice, it is common to use a slight excess of acetic anhydride e.g., a 1.1:1 or 1.2:1 molar ratio to ensure complete conversion of salicylic acid to acetylsalicylic acid and to maximize the yield. This excess can be easily removed from the reaction mixture by hydrolyzing it to acetic acid and then evaporating it during the purification process.