The optimal ratio of aniline to nitric acid needed to achieve the highest yield of p-nitroaniline in the preparation process is 1:1. This is because the reaction between aniline and nitric acid is a one-to-one reaction, as shown in the balanced chemical equation:C6H5NH2 aniline + HNO3 nitric acid C6H5NO2 p-nitroaniline + H2O water By using a 1:1 ratio of aniline to nitric acid, you ensure that both reactants are consumed completely, maximizing the yield of p-nitroaniline. However, it is important to note that the reaction should be carried out in the presence of a suitable acid, such as sulfuric acid, to prevent the formation of unwanted byproducts.