0.1 Colligative properties and ice cream  (Page 10/3)

Colligative Properties and Ice Cream

Objectives

• To record facile and fast data collection from the computer interface, ubiquitous in industry and, in this case, to calculate the molecular weight of the unknown solute using freezing point depression
• To learn the definition of molality and the importance of molality in colligative property calculations
• To learn to calculate the molality of a solution
• To measure the freezing point depression caused when adding antifreeze to tert-butanol
• To calculate the molecular weight of the unknown solute using freezing point depression

Grading

You will be determined according to the following:

• Pre-lab (10%)
• Lab Report Form (80%) – including temperature plots
• TA evaluation of lab procedure (10%)

Introduction

Although colligative properties involve solutions, they do not depend on the interactions between the solvent and the solute molecules but rather on the number of solute particles dissolved in solution. Colligative properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. In this experiment you will explore freezing point depression using a solution of ethylene glycol in tert-butanol. You will then use freezing point depression to calculate the molar mass of an unknown solute that is dissolved in tert-butanol.

Ethylene glycol, (CH2OH)2 the major component of antifreeze, is a large organic molecule that dissolves easily in water. The structure of ethylene glycol is shown in Figure 1.

Antifreeze keeps the water in a car's radiator from freezing because the ethylene glycol molecules get in the way when water tries to crystallize into ice. It is more difficult for the ice crystals to form, due to the fact that the water must be at a lower kinetic energy. Therefore, the water freezes at a lower temperature than if the glycol molecules were not present. The effect of the ethylene glycol molecules present in a solutioncan be quantified by the following equation:

ΔT = iKfm Equation 1

where Δ T = Tpure - Tsolution, the difference between the freezing temperature of the pure solute and the freezing temperature of the solution. Kf is the freezing point depression constant of the solvent, having units of °C/m, and m is concentration of the solution using units of molality. This equation reflects the fact that a more concentrated solution results in a greater change in freezing temperature.

Most of the previous work that we have done with solutions probably has involved units of molarity, or moles per liter of solution. Freezing point depression calculations (as well as those for boiling point elevation) use molality, or moles of solute per kilogram of solvent. By definition, a freezing point depression or boiling point elevation involves a change in temperature. When the temperature of a solution changes, its volume also changes. Since molarity depends on the volume of the solution, a change in temperature will change the solution's molarity. Molality depends on the mass of the solvent, and this does not change with temperature.