5 Steps to a 5 AP Chemistry

186  Step 4. Review the Knowledge You Need to Score High

A 1.00 molal aqueous solution of trichloroacetic acid (CCl 3 COOH) is heated to the

boiling point. The solution has a boiling point of 100.18°C.

Determine the van’t Hoff factor for trichloroacetic acid (Kbfor water =0.512 K kg mol-1).

ΔT=(101.18 – 100.00) =0.18°C =0.18 K

i=ΔT/Kbm=0.18 K/(0.512 K kg mol-1)(1.00 mol kg-1) =0.35

A common mistake is the assumption that the van’t Hoff factor must be a whole number.

This is true only for strong electrolytes at very low concentrations.

Osmotic Pressure

If you were to place a solution and a pure solvent in the same container but separate them

by a semipermeable membrane(which allows the passage of some molecules, but not all

particles) you would observe that the level of the solvent side would decrease while the solu-

tion side would increase. This indicates that the solvent molecules are passing through the

semipermeable membrane, a process called osmosis. Eventually the system would reach

equilibrium, and the difference in levels would remain constant. The difference in the two

levels is related to the osmotic pressure. In fact, one could exert a pressure on the solution

side exceeding the osmotic pressure, and solvent molecules could be forced back through

the semipermeable membrane into the solvent side. This process is called reverse osmosis

and is the basis of the desalination of seawater for drinking purposes. These processes are

shown in Figure 13.1.

Δ

()

TiKm= b = 2(0.512K kg mol–1)

15 00.

250

gNaCl

..

1moleNa l

58.44 g Na l

0 g 1

C

C

kg

1000 g

()

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝⎝

⎜

⎞

⎠

⎟

⎡

⎣

⎢

⎢

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

⎥

⎥

=1.05K

Tbp=(373.15+1.05)K =374.20K (=101.05 C)°

π

Osmotic

pressure

Pure Solution

solvent

Semipermeable

membrane

Net

movement

of solvent

Figure 13.1 Osmotic pressure.

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