Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Icecream on May 15, 2005, 08:20:51 AM

hey guys can any1 help me with these 2 questions:
1. In Breckenridge, Colorado, the typical atmospheric pressure is 520 torr. What is the boiling point of water (delta H[vap] = 40.7kJ/mol) in Breckenridge?
(i thought of using ln(P[vap]) = deltaH[vap] / R x (1/T) + C where the pressure of the water at boiling point is equal to the external pressure but i don't know what C is.)
2. The enthalpy of vaporisation of mercury is 59.1kJ/mol. The normal boiling point of mercury is 357degrees C. What is the vapour pressure of mercury at 25 degrees C?
(i thought of using the same formula for this question but i don't seem to have enough information.)
i really think i must be either doing something really wrong or using the wrong formula to start with!!! can some plz help me, i'm really stuck and i've tried these questions soooo many times!

answer to the first is 89.59 celsius

how come, can u plz explain?

Use the Clausius Clapeyron equation, which is derived from the Clapeyron equation for equilibria between phases.
ln(p2/p1)= (deltaH/RT1) (deltaH/RT2)
In your problem,
p2=760 torr
T2=373 K
p1=560 torr
deltaH= 40700
R=8.31
Calculate for T1.
The second problem can be solved similarly.I hope i calculated correctly, the principle is right.
The Clapeyron eqn is:
dp/dT= deltaS/deltaV
for condensed phasevapour equilibria, deltaV = molar volume of gas since, volume of gas>> volume of solid or liquid.
also, deltaS=deltaH(vap)/T, at boiling point
then use the ideal gas eqn for 1 mole, you get the ClausiusClapeyron eqn.

thanks heaps! i get it now!

anytime