For the chemical reactiom to be at equilibrium:
1- The rate of forward reaction must be equal to the rate of the reverse reaction.
2- The mass of EACH element must be equal before and after the reaction (no NET change in mass), otherwise the equilibrium will shift.
Important note: you need to check the mass of each element before and after the reaction (i.e, reactants side and products side) and the not the mass of the system as a whole. This is because the mass of the whole system will be preserved whether the system is at equilibrium or not (this is the fundamental law of mass conservation)