Concise Physical Chemistry

c09 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

PROBLEMS AND EXAMPLES 141

A

B C

FIGURE 9.14 A ternary phase diagram in which B and C are partially miscible. Figure 9.13

might approach this form at higher temperatures.

Starting with a solution of 0.5 mol of A and 0.5 mol of B at a constant temperature

corresponding to that of phase diagram Fig. 9.14, we add component C in small

portions. What happens?

Solution 9.2 The phase behavior is somewhat complicated. At the first few small

increments of component C, a clear homogeneous solution of A, B, and C results,

corresponding to points on a straight line from the midpoint of axis AB in the

direction of apex C. Soon the coexistence curve is crossed and the solution splits into

two phases. The overall composition continues along the straight line toward C as

increments are added, but the system now consists of two phases corresponding to the

end points of the tie line. At first, a minute amount of the second phase appears; but

further along in the addition process, substantial amounts of both phases are present.

Their compositions always correspond to the end points of the tie line as it intersects

the coexistence curve.

There comes a time when the short end of the tie line approaches the overall

composition line. The amount of each phase is in the inverse ratio of the length

of the tie line cut off by the overall composition line such that the AC-rich phase

predominates over the AB rich phase. Ultimately the coexistence curve is crossed

again and the solution clears, its composition corresponding to the points on the lower

right of the overall composition curve approaching the C apex.

Problem 9.1

(a) How many components are there in a dilute solution of sodium acetate NaAc

in water?

(b) A drop of HCl is added to the solution in part a. Now the anions of the weak

acid Ac−and the strong acid Cl−are competing for the protons H+.How

many components are there in this system of Na+,Ac−,H+,Cl−,H 3 O+, and

a minimal concentration of OH−?

Problem 9.2

(a) What is the effect of a decrease in atmospheric pressure on the freezing and

boiling points of water?