General Chemistry Principles, Patterns, and Applications, 2011

ln(0.7457 torr0.0888 torr) = -DHvap8.314
J / (mol × K)[1(120 + 273)K -1(80.0 + 273) K]
ln(8.398) = -DHvap8.314 J × mol -
1× K -1(-2.88 ´10 - 4K -1)
B We can now use this value of ΔH vap to calculate the vapor pressure of the liquid (P 2) at 160.0°C (T 2):
Using the relationship e ln x = x, we have
2.13 = -DHvap(-0.346 ´10 - 4) J -1× molDHvap
= 61,400 J / mol = 61.4 kJ / mol
ln(P20.0888 torr) = -61,400 J × mol -18.314 J ×mol -1× K -1
[1(160 + 273) K -1(80.0 + 273) K] = 3.86
ln(P20.0888 torr) = 3.86
P20.0888 torr = e3.86 = 47.5
P2 = 4.21 torr
At 160°C, liquid Hg has a vapor pressure of 4.21 torr, substantially greater than the pressure at 80.0°C, as
we would expect.
Exercise
The vapor pressure of liquid nickel at 1606°C is 0.100 torr, whereas at 1805°C, its vapor pressure is 1.000
torr. At what temperature does the liquid have a vapor pressure of 2.500 torr?
Answer: 1896°C
Boiling Points
As the temperature of a liquid increases, the vapor pressure of the liquid increases until it equals the
external pressure, or the atmospheric pressure in the case of an open container. Bubbles of vapor begin to
form throughout the liquid, and the liquid begins to boil. The temperature at which a liquid boils at
exactly 1 atm pressure is thenormal boiling point of the liquid. For water, the normal boiling point is
exactly 100°C. The normal boiling points of the other liquids in Figure 11.16 "The Vapor Pressures of
Several Liquids as a Function of Temperature" are represented by the points at which the vapor pressure
curves cross the line corresponding to a pressure of 1 atm. Although we usually cite the normal boiling
point of a liquid, the actual boiling point depends on the pressure. At a pressure greater than 1 atm, water
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