Mass Balance Equations (MBE) states that the quantity of all species in a solution containing a particular atom must be equal to the amount of that atom delivered to the solution. Dissociation of diprotic acid, H2S, in H2O occurs in a stepwise manner. In this video, acid dissociation beginning with H­2S dissociating to HS- and then to S2- will be animated. In addition, subsequent base back dissociations are also shown. MBEs are written for the dissociation of H2S in H2O.