The optimal ratio of salicylic acid and acetic anhydride to produce the highest yield of aspirin in the synthesis process is 1:1, based on their stoichiometry in the balanced chemical equation:C7H6O3 salicylic acid + C4H6O3 acetic anhydride C9H8O4 aspirin + C2H4O2 acetic acid However, in practice, it is common to use a slight excess of acetic anhydride e.g., 1:1.1 or 1:1.2 to ensure complete conversion of salicylic acid to aspirin and to compensate for any potential side reactions or losses during the synthesis process.