Concise Physical Chemistry

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

184 SOLUTION CHEMISTRY

Α Β

XB

p

pB

pA

pBo

pAo

FIGURE 12.2 Partial and total pressures for a Raoult’s law solution.

Normally the component with the smaller mole fraction is called thesolute, and the

one with the larger mole fraction is called thesolvent. If we are looking at solutions

over a wide range of mole fractions, this designation becomes rather arbitrary. Real

solutions that come very close to satisfying the Raoult’s law idealization are solutions

of toluene in benzene.

The sum of two linear functions is a linear function, so the total vapor pres-

surepcontributed by both components anywhere along the concentration axis is

a linear function of the difference between the vapor pressures of the pure com-

ponents

p=p◦A+XB

(

p◦B−p◦A

)

12.3 A DIGRESSION ON CONCENTRATION UNITS

Concentration units in solution chemistry are simple but rather convoluted at times,

and they can cause unnecessary errors. This section is in part a summary of con-

centration units and notation already used and in part an introduction to the unit

of molality, which is favored in the study of chemistry in solvents other than the

“universal” solvent water. Molality is also used in the study of concentrated solutions

and in very precise work.

We have already been introduced to the mole fraction (Section 1.5), which is the

number of moles of a selected component of the solution relative to the total number

of moles of all components in the solution

Xi=

ni

ni+

∑

nj