The vapor pressures (in torr) of solid and liquid chlorine are given by ln ( P s ) = 24.320 − 3777 K T ln ( P l ) = 17.892 − 2669 K T where T is the absolute temperature. Calculate the temperature and pressure at the triple point of chlorine.

Triple point is that temperature and pressure for a substance at which the chemical substance coexists in liquid, solid and gaseous state with mutual equilibrium.

At this point the equilibrium vapor pressure of solid, liquid and gas phases are mutually equal.

Given expressions of vapor pressure:

[tex]ln (P_s)=24.320 -\frac{3777 K}{T}[/tex] ........................................(1)

[tex]ln (P_l)=17.892 -\frac{2669 K}{T}[/tex] ..........................................(2)

Equating the vapor pressure of two phases of chlorine:

[tex]24.320 -\frac{3777 K}{T}=17.892 -\frac{2669 K}{T}[/tex]

[tex]T=172.37\ K[/tex]

The vapor pressure at this temperature:

[tex]ln (P_s)=24.320 -\frac{3777 }{172.37}[/tex]

[tex]P_s=11.1095\ Pa[/tex]