Material Balance on systems involving reactions is a very important subject matter in Process Calculations. There are different ways in which it can be done. This video explains with an example how to perform material balance on reactive systems using molecular species balance. The problem taken is an unsolved question 4.42 of Felder's book. Initial part of the video teaches you how to calculate degree of freedom when doing species balance on reactive systems. Then it teaches a very important concept of how to choose a basis in order to make your calculations simpler. With the help of this problem, once again the concept of scaling the flowchart was revised. Once the balanced flowchart was obtained, it was figured out which reactant is limiting and which one is in excess and what is the percentage excess. Also the extent of reaction was calculated.