Answer:
Because of the reaction stoichiometry (detailed information given below).
Step-by-step explanation:
Well, reaction stoichiometry determines relationships between reactants and products in a chemical reaction. For example, if the relationship between reactants and/or products is 1 to 1 (e.g. CO + H2O => CO2 + H2), the product is produced as much as the limiting reactant(s) (which is less than the other one in amount) consumed. For example, if 3 moles CO mixed with 0.5 moles of O2 (so O2 is limiting reactant in this case), 0.5 CO2 and 0.5 H2 are produced and 2.5 moles of CO and 0 moles of O2 (because all of O2 consumed) are left over. So, it can be concluded that the reaction stoichiometry has a direct relationship with the amount of the product and the amount of each reactant left over.