Molar Mass and
Freezing Point Depression
Lab Prep
Key Points

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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)
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Constants
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Kf molal freezing point depression constant

Kb molal boiling point elevation constant

m = ΔTf / Kf