Boiling Point of Water
VCL 11-3: The Boiling Point of Water at High Altitude
The relationship between the equilibrium vapor pressure of a liquid or solid and temperature is given by
the Clausius-Clapeyron equation. In this assignment you will measure the vapor pressure of water at a
given temperature and use this data and the Clausius-Clapeyron equation to calculate the boiling point of
water on the top of Mt. Denali in Alaska.
1. Start Virtual ChemLab and select The Boiling Point of Water at High Altitude from the list of
assignments. The lab will open in the Gases laboratory.
2. The balloon is filled with 0.40 moles of water vapor at a pressure of 1500 Torr and a temperature of
400 K. Pull down the lever on the Temperature LCD controller until the temperature stops
decreasing. This temperature represents the equilibrium temperature where water as a gas exists in
equilibrium with water as a liquid. The pressure at this temperature is the vapor pressure. Record the
vapor pressure (in Torr) and the temperature (in K) in the data table.
Data Table
Vapor Pressure (Torr)
Temperature (K)
3. The Clausius-Clapeyron equation may be written in several forms. For this assignment, the most
useful form can be written as
ln
∆H vap ⎛ 1 1 ⎞
P2
⎜ − ⎟
=−
P1
R ⎜⎝ T2 T1 ⎟⎠
If P1 and T1 are the experimental vapor pressure and temperature that you measured and the pressure
at the top of Mt. Denali, P2, is 340 Torr, the boiling point of water, T2, at the top of the mountain can
be calculated by solving the Clausius-Clapeyron equation for T2.
Calculate the boiling point of water at the top of Mt. Denali. The value of R is 8.314 J⋅K-1⋅mol-1 and
∆Hvap for water is 40.67 kJ/mol
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