Solution:
The excess Gibbs energy for a binary system consisting of two liquids is given as $\displaystyle \frac{G^E}{RT} = A \cdot x_1 x_2$, where $A$ is a function of temperature only and $x_i$ is the mole fraction of the component $i$. If this binary system always phase–separates into two coexisting phases during $0.3 \leq x_1 \leq 0.7$, please determine the value of $A$.
