Thermodynamics

Phase diagram of an ideal solution at fixed temperature

The following plot shows the vapor-liquid phase diagram for a binary ideal mixture (components: A and B). The vapor pressures of the pure substances are [math]p_A^*[/math] and [math]p_B^*[/math], respectively.

The blue curve shows the vapor pressure [math]p[/math] of the mixture as a function of the mole fraction of A in the liquid [math]x_A^l[/math]:

[math]p=p_B^*+(p_A^*-p_B^*)x_A^l[/math]

The red curve shows the vapor pressure [math]p[/math] of the mixture as a function of the mole fraction of A in the vapor [math]x_A^v[/math]:

[math]p=\dfrac{p_A^*p_B^*}{p_A^*-(p_A^*-p_B^*)x_A^v}[/math]

In the example below [math]p_A^*=1[/math] (a.u.) and the value of [math]p_B^*[/math] can be changed moving the slider below.

The number of moles in each phase (liquid or vapor) can be obtained from the lever rule:

[math]n^lBK=n^vKR}[/math]