Freezing Point Depression and Boiling Point

COLLIGATIVE PROPERTIES:FREEZING POINT DEPRESSION AND BOILING POINT ELEVATIONDAY 1 – 04 FEBRUARY 2015

Colligative Properties

Depends on the NUMBER of solute, not on the nature of solute particles

Freezing Point Depression Boiling Point ElevationVapor Pressure LoweringOsmotic Pressure

Electrolyte and Non- electrolytes 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 Depression

Freezing Point Depression

Adding a solute to a solvent decreases freezing point of the solvent

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

Tf = Tf° - Tf

Amount of solute

Freezing Point of solvent

Freezing Point Depression

Temperature

Freezing Point Depression constant (1.860 C /m)

Molality (mol/kg)

Formula (non-electrolytes):

Freezing Point Depression

ΔTf freezing point temperature

i van ‘t Hoff factor (sum of subscripts)

Kf Freezing Point Depression constant (1.860 C /m)

m Molality (mol/kg)

ΔTf = i Kf m

Formula (electrolytes):

Freezing Point DepressionEXAMPLES

ΔTf freezing point temperature

i van ‘t Hoff factor (sum of subscripts)

Kf Freezing Point Depression constant (1.860 C /m)

m Molality (mol/kg)

NaCl

Na Cl 1 + 1

i = 2

CaCl2

Ca Cl2

1 + 2

i = 3

Na3PO4

Na3 (PO4) 3 + 1

i = 4

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:

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

Tb = Tb° + Tb (+)

Amount of solute

Boiling Point Elevation

Boiling Point Elevation

Boiling point of elevation

Molal boiling elevation constant (0.5120 C /m)

Molality (mol/kg)

Formula (non-electrolyte):

Boiling Point Elevation

ΔTb freezing point temperature

i van ‘t Hoff factor (sum of subscripts)

Kb Molal Boiling Point Elevation constant (0.5120 C /m)

m Molality (mol/kg)

ΔTb = i Kb m

Formula (electrolytes):

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