LAB VI. MOLAR MASS BY FREEZING POINT DEPRESSION
Colligative properties of a solution are properties that depend on the number of solute particles dissolved.
Colligative properties of a solution include:
◦ Vapor pressure lowering,
◦ Boiling point elevation,
◦ Freezing point depression, and
◦ Osmotic pressure.
The molar mass of a solute may be determined by measuring a colligative property of a solution.
In this experiment the freezing point of a solvent would have been determined from a cooling curve.
Practice problems:
• Calculate the freezing point of a solution of 800 g of ethylene glycole (MW = 62.02 g/mole) in 3,000 g of water given that the Kf(H2O) = 1.86 °C/m.
• How many moles of ethanol are dissolved in t-butanol if the change in the freezing point is 10 °C, and t-butanol's Kf is 8.37 °C/m? How many grams of ethanol are dissolved in this solution?
• Calculate the freezing point of a solution of 200 g of glucose (MW = 180.096 g/mole) in 2,500 g of acetic acid, given that Kf(acetic acid) = 3.90 °C/m, and T°f(acetic acid) = 16.6 °C.
• Calculate the boiling point of a solution of 280 g of glucose (MW = 180.096 g/mole) in 3,600 g of acetic acid, given that Kb(acetic acid) = 3.07 °C/m, and T°b(acetic acid) = 118.5 °C.
• Calculate the boiling point of a solution of 15 g of KBr dissolved in 350 g of water, given that Kb(H2O) = 0.52 °C/m.