Transcript 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
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