Question

A certain liquid has a vapor pressure of 92.0 Torr at 23.0 �C and 315.0 Torr at 45.0 �C. Calculate the value of ?H�vap for this liquid and calculate the normal boiling point of this liquid in �C.

Answer 1

Recall that in equilibrium; especially in vapor-liquid equilibriums, we can use Clasius Clapyeron combination equation in order to relate two points in the same equilibrium line.

The equation is given as:

ln(P2/P1) = -dHvap/R*(1/T2-1/T1)

Where

P2,P1 = vapor pressure at point 1 and 2

dH = Enthalpy of vaporization, typically reported in kJ/mol, but we need to use J/mol

R = 8.314 J/mol K

T1,T2 = Saturation temperature at point 1 and 2

Therefore, we need at least 4 variables in order to solve this.

Substitute all known data:

ln(P2/P1) = -dHvap/R*(1/T2-1/T1)

Change negative signs

ln(P2/P1) = dHvap/R*(1/T1-1/T2)

ln(315/92) = dHvap/8.314*(1/(23+273) - 1/(45+273)

Hvap = ln(315/92)*8.314 / (1/(23+273) - 1/(45+273) = 43781.2376 J/mol = 43.7 kJ/mol

so

b)

Tnormal:

ln(P2/P1) = dHvap/R*(1/T1-1/T2)

ln(760/92) = 43781.2376 /8.314*(1/(23+273)-1/T2)

-1/T2 = ln(760/92) /43781.2376 *8.314 - 1/(23+273)

T2 = (--0.0029774)^-1

T2 = 335 K

T2 = 62°C