Expt 3 - Tro Determination of Molar Mass by Freezing Point Depression - 1034_F18 (1).pdf

Expt 3 - Tro Determination of Molar Mass by Freezing Point Depression - 1034_F18 (1).pdf

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1 DETERMINATION OF MOLAR MASS BY FREEZING POINT DEPRESSION ADDITIONAL READING The concepts in this experiment are also discussed in section 12.6 of Principles of Chemistry – A Molecular Approach , by Tro. ABSTRACT This experiment is divided into three parts. In Part A, you will determine the freezing point of a pure sample of lauric acid by measuring the temperature as the liquid cools and freezes to a solid. Temperatures will be measured and recorded using the Vernier system equipped with a temperature probe, and the data analyzed using features built into the instrument. In Part B, you will be provided with a sample of an unknown compound, and using the same methods as in Part A, determine the freezing point of the mixture, the freezing point depression, and the molar mass of the unknown. It is highly recommended that you read the textbook sections referenced in the above “Additional Reading” section. The information contained therein will provide a strong foundation for understanding the concepts learned during this experiment. BACKGROUND When we dissolve a nonvolatile solute in a liquid, we obtain a solution. The liquid in which the solute dissolves is the solvent. The presence of a solute affects the freezing behavior of the solvent, because the individual solute particles interfere with the solvent-solvent intermolecular interactions that would establish the freezing behavior of the solvent if it were pure. Hence, the freezing point of the solvent is depressed, or lowered, by the presence of solute particles. The identity of the solute is not as important as the number of solute particles in solution. If the solute and solvent react, new substances are formed resulting in a complex system that is not as easily interpreted as the one we are discussing. For our purposes, we will assume no reaction occurs between solute and solvent. The extent of freezing point depression, caused by the presence of solute particles, is different for each solvent, but is always dependent on the molality, m , of the solute. Molality is the number of moles of solute in one kilogram of solvent: kg solvent, of mass solute of moles = m (1) The proportionality constant relating the change in freezing point, D T f , to the molality of solute particles ( m ) is the freezing point depression constant, K f . The mathematical expression for this relationship is shown below, where T f is the freezing point of the pure solvent and T f is the freezing point of the solution. D T f = T f – T f = i x m x K f (2) The van’t Hoff factor, i , is used for ionic solutes, and is related to the number of particles in solution. For NaCl, the expected value of i is equal to 2 (one Na + cation, one Cl anion). In this experiment we will be using solutes that are nonelectrolytes with a value of i equal to 1.
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2 Freezing point measurements are easy to make. We can use the relationship expressed in Equation 1 to determine the molar mass of an unknown. First, we determine the freezing point of a carefully weighed amount of solvent.
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  • Spring '12
  • MICHEL
  • Mass, Carboxylic acid, Freezing-point depression, Lauric acid

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