Chem 125
Quiz #1 Ben
Dr. Retsek
Winter 2008
NAME:
___
KEY
_____________________________
1.
(10 points)
Using the graph below for water answer the following two questions.
Linear Relationship between Pressure
and Temperatue for Water
y = -5220x + 20.7
R
2
= 0.9999
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0025
0.0030
0.0035
0.0040
1/Temperatue (K
-1
)
ln P
vap
(P in torr)
a. What is the heat of vaporization (in kJ/mole) for water?
Graph is Clausis-Clapeyron graph:
ln P = (-
∆
H
vap
/R)(1/T) + C
so slope = (-
∆
H
vap
/R)
-5220 = -(
∆
H
vap
/R) = -(
∆
H
vap
/8.314 J/mole·K)
∆
H
vap
= 433399 J/mole x (1 kJ/1000 J) = 43.4 kJ/mole
b. Calculate the vapor pressure of water at 50
°
C?
Just use equation, but T needs to be in Kelvin.
ln P (or y) = -5220 (1/323 K) + 20.7
ln P = 4.53
P = 93.6 torr
(units were given on y-axis of graph)

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