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 temperatureIf the mass of the solvent is known then

the moles of solute can be calculated:moles of solute = (m) (kg of solvent)

Constants

Kf molal freezing point depression constant

Kb molal boiling point elevation constant

m = ΔTf / Kf