The optimum ratio of aniline to nitric acid for the preparation of p-nitroaniline in the chemical synthesis process is 1:1. This is because the reaction between aniline C6H5NH2 and nitric acid HNO3 is a one-to-one reaction, as shown in the balanced chemical equation:C6H5NH2 + HNO3 C6H5N NO2 H + H2OIn this reaction, one mole of aniline reacts with one mole of nitric acid to produce one mole of p-nitroaniline and one mole of water. To achieve the highest yield and minimize side reactions, it is essential to maintain a 1:1 molar ratio of aniline to nitric acid.