If the composition of the system does not change with time, the system is said to be in chemical equilibrium. itis the state in which net reaction of a system Is zero. in another words we can say that in reversible reactions, a stage is reached when the rate of transformation of reactants into products equals the rate of transformation of products into reactants. At this stage, the composition of reactants and products does not change with time. This does not mean that the reaction has ceased, as both reverse and forward reactions are still taking place but with equal speed. Such equilibria are called dynamic equilibria, Let us consider a reaction of the type A_(g) + B_(g) iff C_(g) + D_(g) K_c = ([C][D])/([A][B]) where K_c is equilibrium constant when the ratio of the concentrations of the product to reactants K_p = (P_C xx P_O)/(P_A xx P_B) where K_P is the equilibrium constant for the ratio of partial pressure of products to reactants. The relation between K_P and K_C is as follows K_P = K_C(RT)^(Deltan) The mass ratio of steam and hydrogen is found to be 1:2 at equilibrium. 3 Fe(s) + 4H_2O(g) iff Fe_3O_4(s) + 4H_2(g) The value of equilibrium constant (K_C) for the above reaction is