Page 109 - Instant notes

D2
SOLUTIONS

Key Notes
An ideal solution is a mixture of two species, A and B, which
show similar molecular interactions between molecules of A,
molecules of B and molecules of B and A. By definition, an ideal
solution obeys Raoult’s Law, , where p i , x i and are
the partial vapor pressure, the mole fraction and the vapor
pressure of liquid species i, and i is either A or B.
The vast majority of solutions are non-ideal solutions and show
deviation from Raoult’s law. In this case, for dilute solutions of a
solute B in a solvent A, termed an ideal-dilute solution, Raoult’s
law applies to the solvent. For the solute, the partial vapor
pressure, p B , is related to its mole fraction, x B , by Henry’s law:
p B =K B x B
where K B is the Henry’s law constant, which quantifies the
deviation from ideal behavior of B. K B is a constant for a
particular solute B in a particular solvent, A.
Related topics Free energy (B6) Colligative properties (D3)
Fundamentals of equilibria (C1)

Ideal solutions

The mole fraction of any species in a liquid system is the equivalent variable to the partial
pressure of a species in a gas, as in each case increasing this variable causes an increase
in the number of molecules of the species per unit volume. This means that the activity of
a liquid is related to its mole fraction. An ideal solution of a mixture of two liquids, A
and B, is one in which the interactions between similar pairs of molecules, A and A or B
and B in a solution are similar in magnitude to those between the dissimilar molecules A
and B. A good example is benzene and toluene, which are molecules with very similar
sizes and shapes and have very similar interactions. In this case Raoult’s law is obeyed,
which is:

where p i, x i and are the partial vapor pressure, the mole fraction and the vapor
pressure of liquid species i, where i is A or B. The vapor pressure of species i is the
pressure of gas i in equilibrium with the pure liquid species, i (see Topic C1). The partial
vapor pressure of liquid i is therefore the partial pressure of i in the vapor mixture in