$17.$ The $\_-$Duhem equation can be used to calculate one component's activity coefficient from the known activity coefficient of another in a binary mixture.

$18.$ Theoretical calculations of activity coefficients often involve the use of models like $\_$ or Wilson equations.

$19.$ Fugacity and activity coefficients play a crucial role in the $\_$ of phases in equilibrium.

$20.$ The concept of excess free energy is pivotal in understanding the non$-$ideal behavior of mixtures, especially in calculating $\_$ coefficients.