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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.
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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)
Constants • Kf molal freezing point depression constant • Kb molal boiling point elevation constant • m = ΔTf / Kf