FPDep - 1 Experiment 13: Determination of Molecular Weight...

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1 Experiment 13: Determination of Molecular Weight by Freezing Point Depression Objective: In this experiment, you will determine the molecular weight of a compound by measuring the freezing point of a solution of the compound and then comparing the freezing point of that solution to that of the pure solvent. Introduction The properties of a solution differ from those of a pure solvent due to interactions that take place between the solute and solvent molecules. The properties that exhibit such changes are called the colligative properties and include vapor pressure lowering, boiling point elevation, freezing point depression and changes in osmotic pressure (see Tro, section 12.6-12.7 pp 535-550). These properties are dependent only upon the number of particles (ions or molecules) which are dissolved in the solvent and not on the identity of the particles. This experiment will examine the phenomenon of freezing point depression. When a particular solute is dissolved in a solvent, the following expression holds true: Δ T = T f ° – T f = K f m The terms T f ° and T f refer to the freezing-point temperatures of the pure solvent and the solution, respectively. The term “m” indicates the molality of the solution, which is defined as the number of moles of solute per 1000 g of solvent . This quantity is used, rather than molarity, because it is not temperature dependent. The constant, K f , is referred to as the freezing-point-depression constant and is dependent only upon the solvent. The change in temperature is also dependent upon the number of solute particles in solution— the more particles that are present, the larger the change in temperature. For this reason, the above equation is sometimes written as: T f ° - T f = K f i m, where i = the number of solute particles produced per formula unit that dissolves. In a solution containing an electrolyte, each ion is considered to be a particle. You will explore the influence of i on the freezing point depression in Part II of the experiment. In Part I of the experiment, you will use cyclohexane, an organic compound that is a liquid at room temperature, as the solvent. You will be able to determine the molecular weight of an unknown compound by observing the freezing point of a solution of the compound in cyclohexane and comparing it to the freezing point of pure cyclohexane. Calculations 1. The compound cyclohexane has a melting point (or freezing point) of about 6°C. You will obtain a series of temperatures of pure cyclohexane as it cools down from room temperature through its freezing point in an ice bath. These temperatures will be plotted as a function of time. Similarly, you will obtain temperatures of a solution of an unknown compound in cyclohexane as it cools down to the freezing point, which will also be plotted. The plots will look similar to those pictured in Figures 1a and 1b, and the T f ° and T f values can be extrapolated from them as shown on Figure Page 1, Expt. 13 .
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2 2. The molality of a solution can be expressed in terms of the molar mass of the
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This note was uploaded on 12/09/2011 for the course SP 108 taught by Professor Whittenburg during the Summer '11 term at Montgomery College.

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FPDep - 1 Experiment 13: Determination of Molecular Weight...

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