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Colligative_Properties_Lab_2 - Colligative Properties...

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Colligative Properties: Freezing Point Depression and Molecular Weight Name: _______________________________________________________ Introduction: Solutions are homogeneous mixtures that contain two or more substances. The major component is called the solvent , and the minor component(s) are called the solute(s) . Since the solution is primarily composed of solvent, the physical properties of the solution resemble those of the pure solvent. However, some of these physical properties, called colligative properties , are independent of the nature of the solute and depend only upon the concentration of solute particles. Examples of colligative properties include vapor pressure reduction, boiling point elevation, freezing point depression, and osmotic pressure. Pure water freezes at 0.0°C (273 K), boils at 100.0°C (373 K), and exerts a vapor pressure of 23.76 mm Hg at 25.0°C (298 K). These values are altered by the presence of a solute. Your are probably familiar with some common examples of these effects: Antifreeze is used to lower the freezing point and raise the boiling point of your engine coolant (water); and salt is used to melt ice on the roads. These effects are expressed quantitatively by the colligative-property law , which states that the freezing point and boiling point of a solution differ from those of the pure solvent by amounts that are directly proportional to the molal concentration of the solute. In its general form this equation is written: T = K • m where T is the boiling point elevation or freezing point depression, K is a constant that is specific for each solvent, and m is the molality of the solution and is expressed as the number of moles of solute per kilogram of solvent. Some representative constants, boiling points, and freezing points are given in the table below. Solvent Freezing point,°C K fp ,°C Boiling point,°C K bp ,°C CH 3 CO 2 H (acetic acid) 16.6 3.90 118.1 2.93 C 6 H 6 (benzene) 5.4 5.12 80.2 2.53 CHCl 3 (chloroform) -63.5 4.68 61.3 3.63 C 2 H 5 OH (ethyl alcohol) -141 78.4 1.22 H 2 O (water) 0.0 1.86 100.0 0.512 C 10 H 8 (naphthalene) 80.6 6.9 218 5.65 C 6 H 12 (cyclohexane) 6.5 20.0 81 2.79 The equations relating the boiling point elevation and freezing point depression to molality are: T bp = K bp • m • i T fp = K fp • m • i
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where T bp and T fp are the change in boiling point and freezing point respectively, K bp and K fp are the boiling and freezing constants for a particular solvent, and i is the van't Hoff factor which is used to account for the number of particles a given solute generates in solution. Substances are usually classified as ionic or covalent, or more precisely, electrolytes and non electrolytes . Electrolytes dissolve in a solvent to give ions (charged particles) in solution while non electrolytes dissolve to give molecules . A mole of non electrolyte such as ethanol would dissolve to produce a mole of ethanol molecules. However, a mole of sodium chloride (NaCl) would dissolve to form two moles of ions (Na + and Cl - ). Because colligative properties are related to the number of solute particles, we would expect a mole of NaCl to have twice the effect as a mole of ethanol. Therefore, (i =1) for ethanol and other non electrolytes, (i =2) for NaCl, and
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