Freezing Point Depression And Boiling Point

Topics: Freezing-point depression, Van 't Hoff factor, Physical chemistry Pages: 24 (491 words) Published: February 15, 2015
COLLIGATIVE
PROPERTIES:
FREEZING POINT
DEPRESSION AND
BOILING POINT
ELEVATION
DAY 1 – 04 FEBRUARY 2015

Colligative Properties
Depends on the NUMBER of solute, not on the nature of
solute particles
Freezing Point Depression
Boiling Point Elevation
Vapor Pressure Lowering
Osmotic Pressure

Electrolyte and Nonelectrolytes
Electrolytes
•Separates in water forming a
solution that conducts electric
current
•IONIC COMPOUNDS

Non- electrolytes
• does not allow the flow of an electric current
• COVALENT COMPOUNDS

Freezing
Point
Depressio
n

Freezing Point
Depression
Adding a solute to a solvent decreases freezing point of the solvent Amount of
solute

Freezing Point of
solvent

Tf° (pure solvent) > Tf (solution) Freezing point of pure
solvent – 0o
Tf = Tf° - Tf

Freezing Point
Depression
Formula (non-electrolytes):

Temperature

Freezing Point Depression constant
(1.860 C /m)
Molality (mol/kg)

Freezing Point
Depression
Formula (electrolytes):

ΔTf = i Kf m

ΔTf
i

freezing point temperature

van ‘t Hoff factor (sum of
subscripts)

Kf
m

Freezing Point Depression
constant (1.860 C /m)
Molality (mol/kg)

Freezing Point
Depression
EXAMPLES

CaCl2

NaCl
Na
1

Cl
+

i =2

Ca
1

ΔTf
Cl2

+

1

i =3

2

Kf

i

Na PO

3
4
freezing point temperature

Na

(PO )

van ‘t Hoff
3 factor (sum of4
subscripts)

3

+

1

Freezing Point Depression
constant (1.860 C /m)

m

i =4

Molality (mol/kg)

PRACTICE
What is the new freezing point of 200 g
of water (Kf = -1.86 oC) if195 g of
sucrose (C12H22O11) are added to it?

ANSWER
∆Tf = -5.301°C

PRACTICE
What is the molality of barium sulfate
(Ba2SO4) with a freezing a point of
1.12°C?

ANSWER
Molality = 0.20 m

PRACTICE
A solution of 3.39 g of an unknown compound in
10.00 g of water has a freezing point of 7.31°C.
The solution does not conduct electricity. What
is the molar mass of the compound?

ANSWER
Molar mass = 86.26 g/mol

Boiling
Point
Elevation

Boiling Point Elevation
Temperature difference between a solution’s boiling point and a pure solvent’s boiling point
For nonelectrolytes:

Amount of
solute

Boiling Point
Elevation

Tb (solution) > Tb° (pure solvent) Boiling Point of pure
solvent - 100°C
Tb = Tb° + Tb (+)

Boiling Point Elevation
Formula (non-electrolyte):

Boiling point of elevation
Molal boiling elevation constant
(0.5120 C /m)
Molality (mol/kg)

Boiling Point Elevation
Formula (electrolytes):

ΔTb = i Kb m

ΔTb

freezing point temperature

i

van ‘t Hoff factor (sum of
subscripts)

Kb

Molal Boiling Point Elevation

m

Molality (mol/kg)

constant (0.5120 C /m)

PRACTICE
What will be the boiling point of an
aqueous solution containing 55.0 g of
glycerol, C3H5(OH)3, and 250 g of water?
Kb(H2O)= 0.512 °c/m

ANSWER
∆Tb = 1.23°C

PRACTICE
What is the boiling point of a solution
containing 34.3 g of magnesium nitrate
dissolved in 0.107 kg of water?

ANSWER
∆Tb = 3.3°C

PRACTICE
A solution containing 28.4 g of sodium
bromate dissolved in water. Find the
mass of the solvent if the boiling point of
the solution is 8.5°C.

ANSWER
Mass of solvent = 0.034 kg

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