Abstract: According to the method of activity calculation in binary system containing compounds from standard entropy of formation of compounds[1], further using the standard heat of fo rmationof compounds, we deduced the new formula which was also for the compound system. The new formula is\[{\rm{dln}}{{\rm{\gamma }}_{\rm{A}}}{\rm{= - }}\frac{{{\rm{\Delta }}{{\rm{H}}_{\rm{f}}}^{\rm{0}}{{\rm{N}}_{\rm{B}}}}}{{{\rm{R}}{{\rm{T}}^{\rm{2}}}{\rm{(x}}{{\rm{N}}_{\rm{B}}}{\rm{ - y}}{{\rm{N}}_{\rm{A}}}{\rm{)}}}}{\rm{dT - dln}}{{\rm{N}}_{\rm{A}}}\;\;\;\;\;\;\;\;(1)\]here ΔH_f⁰ is standard heat of formation of compounds; x,y.is stoichi-ametric factors of the component A,B respectively; N_A N_B is mole fraction of the components A. B respectively; γ_A. γ_B is activity coefficient of components A,B at liquidus temperature.
In the binary system Au-B₁ the activity values calculated by our fo-mula and by Chou's formula are compared with the measured values. They were very well in agreement on the values.
In this paper the values of the binary system Al-La is calculated using our new formula. The results are discussed briefly.