The optimal molar ratio of phenol to nitric acid for the preparation of 2,4-dinitrophenol is 1:2. This is because one molecule of phenol reacts with two molecules of nitric acid to form 2,4-dinitrophenol, as shown in the balanced chemical equation below:C6H5OH + 2 HNO3 C6H3 NO2 2OH + 2 H2OChanging the molar ratio of phenol to nitric acid can affect the yield and purity of the final product in the following ways:1. If the molar ratio of phenol to nitric acid is less than 1:2 i.e., there is an excess of phenol , the reaction will not go to completion, and the yield of 2,4-dinitrophenol will be lower. Additionally, the excess phenol may contaminate the final product, reducing its purity.2. If the molar ratio of phenol to nitric acid is greater than 1:2 i.e., there is an excess of nitric acid , the reaction may proceed further than desired, leading to the formation of undesired by-products such as 2,4,6-trinitrophenol picric acid . This will also result in a lower yield of 2,4-dinitrophenol and reduced purity of the final product due to the presence of by-products.To maximize the yield and purity of 2,4-dinitrophenol, it is essential to maintain the optimal 1:2 molar ratio of phenol to nitric acid during the reaction. Additionally, proper control of reaction conditions, such as temperature and reaction time, can also help to improve the yield and purity of the final product.