Solutions

PROBLEMS
33. What are the boiling point and freezing point of a O.625m aqueous
solution of any nonvolatile, nonelectrolyte solute
34. What are the boiling point and freezing point of a OAOm solution of
sucrose in ethanol
35. A lab technician determines the boiling point elevation of an aqueous
solution of a nonvolatile, nonelectrolyte to be 1.12°C. What is the
solution's molality
36. A student dissolves 0.500 mol of a nonvolatile, nonelectrolyte solute in
one kilogram of benzene (C 6
H 6 ). What is the boiling point elevation
of the resulting solution
Osmosis and Osmotic Pressure
In Chapter 13, you learned that diffusion is the mixing of gases or liquids
resulting from their random motions. Osmosis is the diffusion of solvent
particles across a semipermeable membrane from an area of higher solvent
concentration to an area oflower solvent concentration. Semipermeable membranes
are barriers with tiny pores that allow some but not all kinds of particles
to cross. The membranes surrounding all living cells are semipermeable
membranes. Osmosis plays an important role in many biological systems such
as kidney dialysis and the uptake of nutrients by plants.
Let's look at a simple system in which a sucrose-water solution is separated
from its solvent-pure water-by a semipermeable membrane. During
osmosis, water molecules move in both directions across the membrane, but
the sugar molecules cannot cross it. Both water and sugar molecules contact
the membrane on the solution side, but only water molecules contact the membrane
on the pure solvent side. Thus, more water molecules cross the membrane
from the pure solvent side than from the solution side.
The additional water molecules on the solution side of the membrane create
pressure and push some water molecules back across the membrane. The
amount of additional pressure caused by the water molecules that moved into
the solution is called the osmotic pressure. Osmotic pressure depends upon
the number of solute particles in a given volume of solution. Therefore,
osmotic pressure is another colligative property of solutions.
Careers Using Chemistry
Renal Dialysis
Technician
People with poorly functioning
kidneys must routinely undergo
dialysis in order to survive. The
process, which removes impurities
from the blood through
osmosis, can be stressful for the
patient. Ifyou enjoy working
with and helping people, and
want to work in a medical setting,
consider being a renal
dialysis technician.
Dialysis technicians prepare
the dialysis equipment and the
patient for treatment, start the
treatment, and monitor the
process. They also make necessary
adjustments, keep records,
and respond to emergencies.
Technicians spend more time
with the patients than the
doctors or nurses. They work
in hospitals, outpatient facilities,
and home-based dialysis
programs.
Assessment
37. Explain the nature of colligative properties.
38. Describe four colligative properties of solutions.
39. Explain why a solution has a lower boiling point
than the pure solvent.
40. Thinking Critically Explain why the colligative
properties described in this section may not apply
to solutions containing volatile solutes. Hint:
Volatile solutes are able to leave the liquid phase
and enter the gas phase.
41. Using Numbers Calculate the boiling point elevation
and freezing point depression of a solution
containing 50.0 g of glucose (C 6
H I2 0 6
) dissolved
in 500.0 g of water.
15.3 Colligative Properties of Solutions 475