# Chemical Forums

## Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Ice-cream on May 15, 2005, 08:20:51 AM

Title: pressure
Post by: Ice-cream 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!
Title: Re:pressure
Post by: ksr985 on May 15, 2005, 08:56:49 AM
answer to the first is 89.59 celsius
Title: Re:pressure
Post by: Ice-cream on May 15, 2005, 09:27:28 AM
how come, can u plz explain?
Title: Re:pressure
Post by: ksr985 on May 15, 2005, 11:20:45 AM
Use the Clausius Clapeyron equation, which is derived from the Clapeyron equation for equilibria between phases.

ln(p2/p1)= (deltaH/RT1)- (deltaH/RT2)

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 phase-vapour 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 Clausius-Clapeyron eqn.
Title: Re:pressure
Post by: Ice-cream on May 16, 2005, 01:09:20 AM
thanks heaps! i get it now!
Title: Re:pressure
Post by: ksr985 on May 16, 2005, 01:24:02 AM
anytime