Solution
In the vaporXheptane0.426Xoctane0.574orXoctane1.0000.426 0.574You should now work Exercise 42.Many dilute solutions behave ideally. Some solutions do not behave ideally over the
entire concentration range. For some solutions, the observed vapor pressure is greater
than that predicted by Raoult’s Law (Figure 14-11a). This kind of deviation, known as a
positive deviation,is due to differences in polarity of the two components. On the molec-
ular level, the two substances do not mix entirely randomly, so there is self-association of
each component with local regions enriched in one type of molecule or the other. In a
region enriched in A molecules, substance A acts as though its mole fraction were greater
than it is in the solution as a whole, and the vapor pressure due to A is greater than if the
solution were ideal. A similar description applies to component B. The total vapor pres-
sure is then greater than it would be if the solution were behaving ideally. A solution of
acetone and carbon disulfide is an example of a solution that shows a positive deviation
from Raoult’s Law.
Another, more common type of deviation occurs when the total vapor pressure is less
than that predicted (Figure 14-11b). This is called a negative deviation.Such an effect is
due to unusually strong attractions (such as hydrogen bonding) between unlikemolecules.24.8 torr
43.2 torrPoctane
Ptotal18.4 torr
43.2 torrPheptane
PtotalHeptane (pure vapor pressure
92.0 torr at 40°C) is a more volatile
liquid than octane (pure vapor
pressure31.0 torr at 40°C). Its mole
fraction in the vapor, 0.426, is higher
than its mole fraction in the liquid,
0.200.
560 CHAPTER 14: Solutions
Figure 14-11 Deviations from Raoult’s Law for two volatile components. (a) Positive
deviation. (b) Negative deviation.Vapor pressureP^0 BP^0 AXAXB00
1001(a)Vapor pressureP^0 BP^0 AXAXB00
1001(b)OCCCH 3 CH 3S Sacetonecarbon disulfide