Molar Mass and Freezing Point Depression

Molar Mass and
Freezing Point Depression
Lab Prep
Key Points
It is the number of solute particles, not
their identity, which determines the
magnitude of the boiling-point elevation
and freezing point depression.
Properties based only on the number of
solute particles and not their identity
are known as colligative properties.
Real World Applications
Antifreeze protects a car’s engine from heat
and cold.
Salt is used in making ice cream and on
highways and bridges because it depresses
the freezing point.
Molarity (M)
Number of moles of solute in one liter
of solution
M = mol solute/volume of solution (L)
*Volume varies with temperature
Molality (m)
Moles of solute per kg of solvent
m = mol of solute/kg of solvent (mass)
*Independent of temperature
If the mass of the solvent is known then
the moles of solute can be calculated:
moles of solute = (m) (kg of solvent)
Kf molal freezing point depression constant
Kb molal boiling point elevation constant
m = ΔTf / Kf