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Colligative Properties
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Colligative Properties of Solutions The wood frog is a remarkable creature because it can
survive being frozen. Scientists believe that a substance inthe cells of this frog acts as a natural antifreeze, which
prevents the cells from freezing. You will discover how asolute can change the freezing point of a solution.
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Colligative Properties depend on the concentration (number) of solute
particles but NOT on their identity.
depend on the lowering of the escaping tendency ofsolvent particles by the addition of solute particles.
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Colligative Properties of Solutions What are four colligative properties of
solutions?
Vapor Pressure Lowering Osmotic Pressure
Freezing Point Depression
Boiling Point Elevation
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Vapor-Pressure Lowering In a pure solvent, equilibrium is established between the
liquid and the vapor.
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Vapor-Pressure Lowering In a solution, solute particles reduce the number of free
solvent particles able to escape the liquid. Equilibrium isestablished at a lower vapor pressure.
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Vapor-Pressure Lowering The decrease in a solutions vapor pressure is
proportional to the number of particles the solute makesin solution.
Hence, the vapor pressure of the pure liquid is higher thanthat of the solution.
Psolvent = xsolvent Psolvent
where Psolvent = vapor pressure of the solvent in solution,
xsolvent = mole fraction of the solvent
Psolvent = vapor pressure of the pure solvent.
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Vapor-Pressure Lowering Three moles of glucose dissolved in water produce 3 mol of
particles because glucose does not dissociate.
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Vapor-Pressure Lowering Three moles of sodium chloride dissolved in water produce 6
mol of particles because each formula unit of NaCl dissociatesinto two ions.
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Vapor-Pressure Lowering Three moles of calcium chloride dissolved in water produce 9
mol of particles because each formula unit of CaCl2dissociates into three ions.
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Osmotic PressureWhen two liquids, such
as a solvent and asolution, are separated
by a semipermeablemembrane that allowsonly solvent molecules topass through, then thereis a net transfer ofsolvent molecules fromthe solvent to thesolution. This process iscalled osmosis.
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Osmotic Pressure Osmosis can be stopped by applying pressure to
compensate for the difference in escaping tendencies.The pressure required to stop osmosis is called osmoticpressure.
In dilute solutions, osmotic pressure is directly
proportional to the molarity of the solution and itstemperature in Kelvin.
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Freezing-Point Depression The difference in temperature between the freezing point of a
solution and the freezing point of the pure solvent is the
freezing-point depression.
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Freezing-Point Depression The magnitude of the freezing-point depression is
proportional to the number of solute particles dissolvedin the solvent and does not depend upon their identity.
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Freezing-Point Depression The freezing-point depressionof aqueous solutions makeswalks and driveways saferwhen people sprinkle salt on
icy surfaces to make ice melt.
The melted ice forms asolution with a lower freezingpoint than that of pure water.
Why would calcium chloride(CaCl2) be a better salt forthis purpose than sodiumchloride (NaCl)?
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Boiling-Point Elevation Boiling-Point Elevation
The difference in temperature between the boiling point of asolution and the boiling point of the pure solvent is the
boiling-point elevation. The same antifreeze added to automobile engines to prevent
freeze-ups in winter, protects the engine from boiling over insummer.
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Boiling-Point Elevation The magnitude of the boiling-point elevation is
proportional to the number of solute particles dissolvedin the solvent.
The boiling point of water increases by 0.512C for every moleof particles that the solute forms when dissolved in 1000 g ofwater.
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Molality and Mole Fraction The unit molality and mole fractions are two additional
ways in which chemists express the concentration of asolution.
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Molality and Mole Fraction The unit molality (m) is the number of moles of solute
dissolved in 1 kilogram (1000 g) of solvent. Molality is alsoknown as molal concentration.
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Molality and Mole Fraction To make a 0.500m solution of
NaCl, use a balance tomeasure 1.000 kg of waterand add 0.500 mol (29.3 g) ofNaCl.
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Molality and Mole Fraction Ethlylene Glycol (EG) is added to water as antifreeze.
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Molality and Mole Fraction The mole fraction of a solute in a solution is the ratio of the
moles of that solute to the total number of moles of solvent
and solute.
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Molality and Mole Fraction In a solution containing nAmol of solute A and nB mol of
solvent B (XB), the mole fraction of solute A (XA) and the molefraction of solvent B (XB) can be expressed as follows.
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Freezing Point Depression16 4
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Freezing-Point Depression
and Boiling-Point Elevation
The magnitudes of the freezing-point depressionand the boiling-point elevation of a solution aredirectly proportional to the molal concentration (m),
when the solute is molecular, not ionic.
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E evat on
The constant, Kf, is the molal freezing-point depressionconstant, which is equal to the change in freezing point for a1-molal solution of a nonvolatile molecular solute.
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E evat on
The constant, Kb, is the molal boiling-point elevationconstant, which is equal to the change in boiling point for a 1-molal solution of a nonvolatile molecular solute.
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for Sample Problem 16.8
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for Sample Problem 16.9
http://chemasap/dswmedia/rsc/asap1_chem05_cmps1636.html