To manipulate chemical reactions on a large scale, scientists use stoichiometry to quantify those reactions. The use of stoichiometry ensures there are the right amount of reactants and products. Without it, reactions can be incomplete, with expensive materials wasted and harmful byproducts created. Using stoichiometry, scientists are creating chemicals that take the place of petroleum in fabricating sustainable materials. At a different lab, scientists are mimicking the process of photosynthesis to convert the sun’s energy into storable chemical energy. Part of Chemistry: Challenges and Solutions Series.