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Chemistry B Colligative Properties. © Mr. D. Scott; CHS. In optimal flight conditions, air moves smoothly over the wings of the plane. Deicing system not operating. Cells lie close to airfoil section. Ice is permitted to form.

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slide1

Chemistry B

Colligative

Properties

© Mr. D. Scott; CHS

slide6

In optimal flight conditions, air moves smoothly over the wings of the plane.

Deicing system not operating. Cells lie close to airfoil section. Ice is permitted to form.

Ice on the wings disrupts airflow and adds weight, requiring more airspeed to maintain the same amount of lift.

After deicer system has been put into operation, the center cell inflates cracking ice.

Deicer boot on leading edge of wing

When the center cell deflates, outer cells inflate. This raises the cracked ice causing it to be blown off by the air stream.

slide8

Colligative Properties of Solutions

Colligative properties are the solvent properties that change when a solute is dissolved. They depend only on the number of solute particles in solution and not on the nature (identity) of the solute particles.

Vapor-Pressure Lowering

Boiling-Point Elevation

Freezing-Point Depression

Osmotic Pressure (p)

vapor pressure lowering in a solution
Vapor Pressure Lowering in a Solution

The diagram below shows how a phase diagram is affected by dissolving a solute in a solvent.

The black curve represents the pure liquid and the blue curve represents the solution.

Notice the changes in the freezing & boiling points.

vapor pressure lowering
Vapor Pressure Lowering

The presence of a non-volatilesolute results in reduced evaporation. The bonds between the solute-solvent particles cause more solvent particles to require more energy to change into the vapor phase.

experimenting with vapor pressure lowering
Experimenting with Vapor Pressure Lowering

The liquids in both beakers evaporate but not at the same rate.

The vapor condenses back into both beakers equally.

What is the long-term effect of this difference?

slide12

Example Problem 1

What is the vapor pressure of an aqueous solution containing 100. g of sucrose (C12H22O11) dissolved in 400. g of water at 25.0°C? (Po=23.8 mmHg)

Raoult’s law

mixtures of volatile liquids
Mixtures of Volatile Liquids

Since both liquids evaporate, they both contribute to the vapor pressure. These mixtures result in HIGHER total vapor pressures than would exist when they are separate.

slide14

Boiling-Point Elevation

0

DTb = Tb – T b

0

T b is the boiling point of

the pure solvent

0

Tb > T b

DTb = Kbm i

T b is the boiling point of

the solution

DTb > 0

m is the molality of the solution

Kbis the molal boiling-point elevation constant (°C/m) for a given solvent

iis the van’t Hoff Factor for a given solute

slide15

Freezing-Point Depression

0

DTf = T f – Tf

0

T f is the freezing point of

the pure solvent

0

T f > Tf

DTf = Kfm i

T f is the freezing point of

the solution

DTf > 0

m is the molality of the solution

Kfis the molal freezing-point depression constant (°C/m) for a given solvent

iis the van’t Hoff Factor for a given solute

slide17

Electrolyte Solutions & van’t Hoff Factors

actual number of particles in soln after dissociation

van’t Hoff factor (i) =

number of formula units initially dissolved in soln

Electrolytes divide into ions upon dissolving and result in yielding more particles in solution than their formulas indicate.

Example: NaCl  Na+ + Cl- dissociation results in two ions

Theoretically, the van’t Hoff Factor would be 2.0

The van’t Hoff Factor of 0.0500 M Electrolyte Solutions at 25°C

slide18

Example Problem 2

What is the freezing point of an aqueous solution containing 100. g of NaCl dissolved in 600. g of water?

DTf = Kfm i

Na+/Cl-

Kf = 1.86 C°/m and i = 2.0 (two ions)

Molal concentration must be calculated.

Solution FP = Normal FP – DTf = 0.0 – 10.6 = - 10.6 °C

slide19

Example Problem 3

The Dead Sea is a hypersaline body of water. It has 33.7% salinity. It is 8.6 times more salty than the average ocean or sea. It has an average density of 1.24 Kg/L (374g salt/Kg H2O).

If the salt concentration is 33.7 %w/v and the average molar mass of the salts is 85.4 g/mol, what is the boiling point of this water?

m = mol/Kg

= 4.38 m

Solution BP = Normal BP + DTb

100 °C + 5.69 C° = 106 °C

slide20

Example Problem 4

moles of solute

m=

mass of solvent (kg)

=

3.255 kg solvent

1 mol

62.0 g

1499 g x

What is the freezing point of a solution containing 1499 g of ethylene glycol (antifreeze) in 3255 g of water? The molar mass of ethylene glycol is 62.0 g\mol.

DTf = Kfm i

Kf water = 1.86 0C/m

= 7.43 m

i = 1.0 for all non-electrolytes (molecules that don’t ionize)

DTf = Kfm

= 1.86 0C/m x 7.43 m = 13.8 0C

Solution FP = Normal FP - DTf

= 0.00 0C – 13.8 0C = -13.8 0C

slide22

Molecular Weight by Freezing Point Depression

Example Problem 5

2.00 g of an unknown molecular solute is dissolved in 10.0 g of solvent with Kf = 4.5 C°/m. The DTf is measured to be 5.00 C° (shown at left). What is the molar mass of the solute?

Temperature

DTf

Molar mass = 180. g/mol

Time

osmotic pressure
Osmotic Pressure

Osmosis is the spontaneous movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration

Osmotic Pressure - The Pressure that must be applied to stop osmosis

p = MRTi

where p = osmotic pressure

i = van’t Hoff factor

M = molarity

R = ideal gas constant

T = Kelvin temperature

slide25

Osmotic Pressure

Example Problem 6

The Jubail desalination plant in Saudi Arabia is the largest in the world. The plant, located on the Persian Gulf, produces 800 million gallons per day (50% of Saudi Arabia’s fresh water supply).

If the concentration of salt in the seawater is 0.60 M, what pressure in psi is required to start reverse osmosis (used in desalination)?

Assume 25.0°C, i = 2.0, and 1atm=14.7psi

For comparison, most car tires are inflated to between 45 and 50 psi.

slide26

Red blood cells

(cell membrane only)

Concentration: inside<outside inside=outside inside>outside

Plant cells

(cell membrane & cell wall)