The optimal ratio of terephthalic acid to ethylene glycol in the synthesis of polyethylene terephthalate PET is 1:1. This stoichiometric ratio ensures that both reactants are fully consumed, resulting in the highest yield and quality of the final product. However, in practice, a slight excess of ethylene glycol e.g., 1.05:1 is often used to ensure complete reaction of terephthalic acid and to control the molecular weight of the polymer.