The Foundations of Chemistry

Easily vaporized liquids are called volatileliquids, and they have relatively high vapor
pressures. The most volatile liquid in Table 13-4 is diethyl ether. Water is the least volatile.
Vapor pressures can be measured with manometers (Figure 13-12).
Stronger cohesive forces tend to hold molecules in the liquid state. Methyl alcohol
molecules are strongly linked by hydrogen bonding, whereas diethyl ether molecules are
not, so methyl alcohol has a lower vapor pressure than diethyl ether. The very strong
hydrogen bonding in water accounts for its unusually low vapor pressure (see Table
13-4). Dispersion forces generally increase with increasing molecular size, so substances
composed of larger molecules have lower vapor pressures.

Figure 13-12 A representation of

the measurement of vapor pressure

of a liquid at a given temperature.

The container is evacuated before

the liquid is added. At the instant

the liquid is added to the container,

there are no molecules in the gas

phase so the pressure is zero. Some

of the liquid then vaporizes until

equilibrium is established. The

difference in heights of the mercury

column is a measure of the vapor

pressure of the liquid at that

temperature.

Vapor pressure

at temperature

of measurement

Time

Equilibrium:

liquid and vapor

Ptotal = Pvapor

Initial: liquid only

Figure 13-13 Plots of the vapor

pressures of the liquids in Table

13-4. The normalboiling point of a

liquid is the temperature at which

its vapor pressure is equal to one

atmosphere. Normal boiling points

are: water, 100°C; benzene; 80.1°C;

methyl alcohol, 65.0°C; and diethyl

ether, 34.6°C. Notice that the

increase in vapor pressure is not

linear with temperature.

Methyl

alcohol

Diethyl

ether

Benzene

Water

T ( C)

2000

1500

1000

760

Vapor pressure (torr) 500

0 25 50 75 100

1 atm