The Foundations of Chemistry

EXAMPLE 13-3 Vapor Pressure Versus Temperature

The normal boiling point of ethanol, C 2 H 5 OH, is 78.3°C, and its molar heat of vaporiza-

tion is 39.3 kJ/mol (Appendix E). What would be the vapor pressure, in torr, of ethanol at

50.0°C?

Plan

The normal boiling point of a liquid is the temperature at which its vapor pressure is 760

torr, so we designate this as one of the conditions (subscript 1). We wish to find the vapor

pressure at another temperature (subscript 2), and we know the molar heat of vaporization.

We use the Clausius–Clapeyron equation to solve for P 2.

Solution

P 1 760 torr at T 1 78.3°C
273.2351.5 K

P 2 _?_atT 2 50.0°C
273.2323.2 K

Hvap39.3 kJ/mol or 3.93 104 J/mol

We solve for P 2.

ln    

ln 1.18 so e1.180.307

P 2 0.307(760 torr) 233 torr (lower vapor pressure

at lower temperature)

You should now work Exercise 58.

P 2



760 torr

P 2



760 torr

1



323.2 K

1



351.5 K

3.93 104 J/mol



8.314 mo

J

l K



P 2



760 torr

502 CHAPTER 13: Liquids and Solids

(Enrichment, continued)

TABLE 13-6 General Effects of Intermolecular Attractions on Physical

Properties of Liquids

Volatile Liquids Nonvolatile Liquids

(weak intermolecular (strong intermolecular

Property attractions) attractions)

cohesive forces low high

viscosity low high

surface tension low high

specific heat low high

vapor pressure high low

rate of evaporation high low

boiling point low high

heat of vaporization low high