Term information
Balance equation for mass. "In general, the calculations are rather complicated. For instance, in the case of a dibasic acid, H2A dissolved in water the two reactants can be specified as the conjugate base, A2-, and the proton, H+. The following equations of mass-balance could apply equally well to a base such as 1,2-diaminoethane, in which case the base itself is designated as the reactant A: T_A = [A] + [HA] +[H_2A] \, T_H = [H] + [HA] + 2[H_2A] - [OH] \, With TA the total concentration of species A. Note that it is customary to omit the ionic charges when writing and using these equations. When the equilibrium constants are known and the total concentrations are specified there are two equations in two unknown "free concentrations" [A] and [H]. This follows from the fact that [HA]= 1 T_A = [A] + \beta_1[A][H] + \beta_2[A][H]^2 \, T_H = [H] + \beta_1[A][H] + 2\beta_2[A][H]^2 - K_w[H]^{-1} \, so the concentrations of the "complexes" are calculated from the free concentrations and the equilibrium constants. General expressions applicable to all systems with two reagents, A and B would be T_A=[A]+\sum_i{p_i \beta_i[A]^{p_i}[B]^{q_i}} T_B=[B]+\sum_i{q_i \beta_i[A]^{p_i}[B]^{q_i}} It is easy to see how this can be extended to three or more reagents." source: http://en.wikipedia.org/wiki/Chemical_equilibrium#Mass-balance_equations