Concise Physical Chemistry

c12 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

REAL SOLUTIONS 185

We are often interested in binary solutions in which one substance is clearly the solute

and one is clearly the solvent. An example is a dilute solution of NaCl in water, in

which NaCl is clearly the solute and water is the solvent. It is conventional to use

n 2 to refer to the number of moles of solute while usingn 1 to refer to the number of

moles of solvent. The mole fractions for a binary solution are

X 2 =

n 2

n 1 +n 2

and

X 1 =

n 1

n 1 +n 2

12.4 REAL SOLUTIONS

Few real solutions follow Raoult’s law and none follow it exactly. (This is like saying

that there is no perfectly ideal gas.) Deviations may give vapor pressure curves

that are consistently higher than Raoult’s law as shown in Fig. 12.3, or they may be

consistently lower. Rarely they may be higher at one end and lower at the other so that

the vapor pressure curve crosses the Raoult’s law curve to give a positive deviation

at one end and a negative deviation at the other. Solutions in which attractive AB

forces are weak give a positive deviation from Raoult’s law because molecules leave

the solution more easily than from the pure liquid. Mixtures with strong AB forces

show a negative deviation.

Α Β

XB

p

pB

pA

pBo

pAo

FIGURE 12.3 Consistent positive deviations from Raoult’s law. In this case, intermolecular

attractive forces are less strong in the solution than in the pure components A and B. The

presence of B encourages A to leave the solution and go into the vapor phase.