Colligative Properties Power Point

8/3/2019 Colligative Properties Power Point

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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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amp e ro em


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amp e ro em16.7

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 on16 4


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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 on16 4


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E evat on16.4

E evat on16 4


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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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E evat on16 4


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E evat on16.4

E evat on16 4


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E evat on16.4

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16.8

f S l P bl 16 8


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for Sample Problem 16.8

16 9
http://chemasap/dswmedia/rsc/asap1_chem05_cmps1633.html


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16.9


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for Sample Problem 16.9
http://chemasap/dswmedia/rsc/asap1_chem05_cmps1636.html

Colligative Properties Power Point

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