VAPOUR PRESSUREVapour pressure and total gas pressure
If a sealed vesselcontains a liquid X (such as ethanol or water) and a gas Y (such as
nitrogen), it is usually safe to assume that the gas Y is saturated with the vapour of X.
As expected from Dalton’s law, the total gas pressure above the liquid is the sum of
the gas and vapour pressures. This is illustrated by the following example.
167Example 10.3
A sealed bottle contains carbon monoxide gas (CO) and about
3cm^3 of liquid water at 20C. A pressure gauge measured the
total gas pressure in the bottle as 81.2 kPa. What is the partial
pressure of carbon monoxide in the bottle? The saturated vapour
pressure of water at 20C is 2.33 kPa.Answer
The presence of liquid water in the bottle shows that there is enough water
present to saturate the CO with water vapour (Fig. 10.13). Sincetotal gas pressure 81.2 kPa pCOpH 2 Owhereprepresents the vapour pressure contributions from both substances.
SincepH 2 O2.33 kPa,pCO81.22.3378.9 kPaFig. 10.13Vapour pressure
and total gas pressure.Volatility of solvents
Ethanol evaporates much more readily than water; therefore, we say that ethanol is
morevolatile. This is reflected in Fig. 10.12, where the vapour pressure of ethanol
exceeds the vapour pressure of water at every temperature.
One way to estimate the volatility of solvents at room temperature is to saturate
pieces of filter paper with solvent, and time how long it takes for all the solvent to
evaporate. Alternatively, the boiling point of a solvent may be taken as a rough indi-
cation of its volatility. One of the most volatile solvents available in the laboratory is
ethoxyethane (diethyl ether), with a normal boiling point of 34.5C (see Box 10.3).
Vapour pressure
calculationsA corked test-tube at 25C
contains air and a few
drops of liquid ethanol. The
total gas pressure in the
tube is 101 kPa. What is
the partial pressure of air
in the tube? (Saturated
vapour pressure of ethanol
at 25C is 7.9 kPa.)Exercise 10L