The variation of the Gibbs energy of mixing with composition for two perfect gases at constant temperature and pressure is given by

Where x is the mole fraction of A, n is the total amount of A and B (mol), T is the temperature (kelvin), and R is the gas constant (J K^-1 mol^-1).

Note that the change in Gibbs energy is less than zero for all compositions, which indicates that two gases mix spontaneously in all proportions.

To graph the change in Gibbs energy (J) enter the parameters (i.e., n and T) and hit New Plot. Up to 5 plots can be displayed at one time. The Clear button will remove all plots. To see the parameters for each plot hit the Legend on/off button. The Redraw button will refresh the graph. This is useful when the function domain (i.e., mole fraction of A (x) ) has been changed. To see the value of each plot at a given point, move your cursor to the desired location then click and hold.