Calculate the moles of any of the reactants present by dividing the original quantity of that reactant by the reactant's molecular weight. As an example, we'll use the equation for plant respiration, C6H12O6+6O2 --> 6CO2+6H2O+36ATP, in which C6H12O6 is glucose, O2 is oxygen, H20 is water, and ATP is adenosine triphosphate (energy). If we start with 360 grams of glucose, to obtain the moles of glucose present, we would divide the original quantity of glucose by its molecular weight (180 grams). Thus, we would find two moles of glucose present.
Multiply the moles you obtained in Step 1 by the molar ratio of reactant to CO2. We can obtain the molar ratio from the ratio of the coefficients in the above equation; thus, the ratio in this case is 1 to 6. For each mole of reactant (glucose), that reactant produces 6 moles of CO2. Since our hypothetical reaction involves 2 moles, the reaction produces 12 moles of CO2.
Multiply the moles of CO2 by the molar volume of CO2, since CO2 is a gas. Each mole of CO2 contains 22.4 liters of CO2. Therefore, multiply 12 moles by 22.4 L/mol to obtain 268.8 L CO2.