The total mass of salt leaving the tank equals the product of the concentration in the mixture c3 and the volume flow rate leaving the tank. Because water is actually incompressible, however, that flow rate must be Q1 + Q2. Therefore, the output of salt is c3 X (Q1 + Q2). Because the concept of mass or material balance applies here and output = input, the following relationship is obtained:
c3 X (Q1 + Q2) = c1 X Q1 + c2 X Q2
Solving the above equation for c3 by dividing both sides by (Q1 + Q2), we obtained the following mass balance equation
c3 = (c1 X Q1 + c2 X Q2)/Q1 + Q2
Mass balance calculation can be applied to the natural environmental systems, such as streams, rivers, lakes and even the atmosphere e.g Stream pollution.