A closure procedure based on the probability density functions of the concentrations of chemically inert species has been developed and applied to derive the mathematical forms of the average values of the nonlinear reaction terms in the turbulent mass balances of reacting species. Single and complex (competitive-consecutive and parallel), instantaneous and fast homogeneous chemical reactions have been considered when initially separated reagents mix. The probability density functions of the inert type composition variables formed with reagent concentrations have been approximated with the Beta PDF; the distribution of concentration variance being measured or calculated from the turbulent mixer model previously proposed by the author. Some features of the simple, first order closures for single and complex chemical reactions have been interpreted and explained with the use of the present model. An extensive comparison of the model predictions with experimental results has been performed as well.