Limiting Reactants So far, we have only looked at chemical equations where there is just enough of each reactant to make a product with no reactants left over. In the real world, this rarely happens. It's kind of like if you wanted to make a cake and you had just the right amount of eggs, oil, cake mix, even water in your kitchen. No more. No less. It's the same with chemical reactions. When hydrogen and oxygen react to make water the molar ratio would have to be 2mol of hydrogen gas for every 1 moles of oxygen gas in order make every 2 moles of water. But what if you had 40 moles of alhydrogen gas and 25 moles of oxygen gas? 40 moles of H, would react with 20 out of the 25 mol of O, but you would still have 5 mol of O, that has no H, to react with. You will not produce any more water because you ran out of H2. When this happens we call the substance that does not all react the excess reactant (in this case the 0,) and the substance we ran out of first (in this case the H,) the limiting reactant. The limiting reactant "limits" the amount of product you can produce. A limiting reaction problem would look something like this: How many grams of carbon dioxide Co, forms in the reaction of 25 grams of glucose with 40 grams of oxygen? The first thing you need is a balanced equation: C. H,,0. +60, = 600, + 6H20 Next you need to do 2 (yes, 2!) stoichiometry problems to see which amount would produce the most CO, 25 g of glucose or 40 grams of oxygen. So, for this problem, you would wind up with this.​