Chapter 13

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Chapter 13- Unit 2
Colligative Properties - are properties of solutions that depend on the number of
molecules in a given volume of solvent and not on the properties/identity (e.g. size or
mass) of the molecules
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Vapor-Pressure Lowering

Nonvolatile Substance- A substance that has little tendency to
become a gas under existing conditions.

Adding a nonvolatile solute to a solvent always lowers the
vapor pressure.
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Freezing-Point Depression

Freezing-Point Depression(Δtf)- the difference between the
freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent, and it is directly proportional to
the molal concentration of the solution
Δtf = Kfm
Molal freezing point constant (Kf)- the freezing point depression of
the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte
solute.
Molality (m) – mol solute/kg of solvent
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Freezing-Point Depression Con’t
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Practice Problem

A water solution containing an unknown quantity of a nonelectrolyte
solute is found to have a freezing point of -0.23°C. What is the molal
concentration of the solution?

m=
Δtf / Kf
Δtf = f.p of solution – f.p of pure solvent = -0.23°C – 0.00°C = -0.23°C
m= 0.12m
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Boiling-Point Elevation
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Boiling-Point Elevation- the difference between the boiling
points of the pure solvent and a solution of a non-electrolyte
in that solvent, and it is directly proportional to the molal
concentration of the solution
Δtb= Kbm
*Molal freezing point constant (Kb)
Example - antifreeze
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Osmotic Pressure

Osmosis- The movement of solvent
through a semipermeable membrane
from the side of lower solute
concentration to the side of higher solute
concentration

Semipermeable membrane- allows the
passage of some particles while blocking
the passage of others

Osmotic Pressure- the external pressure
that must be applied to stop osmosis
 Because osmotic pressure is dependent
on the concentration of solute particles
and not on the type of solute particles,
it is a colligative property
 The greater the concentration of a
solution, the greater the osmotic
pressure

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