Abstract: According to the method of activity calculation in binary system containing compounds from standard entropy of formation of compounds1, 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\\rmdln\rm\gamma _\rmA\rm= - \frac\rm\Delta \rmH_\rmf^\rm0\rmN_\rmB\rmR\rmT^\rm2\rm(x\rmN_\rmB\rm - y\rmN_\rmA\rm)\rmdT - dln\rmN_\rmA\;\;\;\;\;\;\;\;(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.