Molecular_Weight_by_Freezing_Point_Depression

# Molecular_Weight_by_Freezing_Point_Depression - General...

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General Chemistry I (FC, 09 - 10) Lab # 13 Molecular Weight Determination by Freezing Point Depression 1 Revised 8/19/2009 I & TRODUCTIO & If a nonvolatile solid compound (the solute) is dissolved in a liquid (the solvent), the vapor pressure of the liquid solvent is lowered. This decrease in the vapor pressure of the solvent results in several easily observable physical changes, including a boiling point elevation and a freezing point depression. Many years ago, chemists observed that at low solute concentrations the changes in the boiling point, the freezing point and the vapor pressure of a solution are all proportional to the amount of solute. These three properties are collectively known as colligative properties of solutions. The colligative properties of a solution depend on the number of solute particles present in a given amount of solvent and not on the kind of particles dissolved. When working with boiling point elevations or freezing point depressions of solutions, it is convenient to express the solute concentration in terms of its molality, m, defined by the relationship in equation 1. solvent kg A mol m A of molality = = (1) The boiling point elevation (T b - T b ° ) or & T b and the freezing point depression (T f ° - T f ) or & T f in ° C at low concentrations are then given by equations 2 and 3. & T b = K b m ( 2 ) & T f = K f m (3) T b and T f are respectively the boiling point and freezing point of the solution. T b ° f ° are respectively the boiling point and freezing point of the pure solvent. The values K b and K f are the boiling point elevation and freezing point depression constants whose value depends on the solvent used. For water, for example, K b is 0.52 ° C/m and K f is 1.86 ° C/m. For benzene, K b is 2.53 ° C/m and K f is 5.12 ° C/m. One of the main uses of colligative properties of solutions is in connection with the determination of the molecular weights of unknown substances. If we dissolve a known amount of solute in a given amount of solvent and measure & T f of the solution produced, we can find the molality from equation 3, as long as the appropriate K f value is also known. From m, moles can be determined. Knowing both grams and moles allows us to calculate a molecular weight. In this experiment, you will be asked to estimate the molecular weight of an unknown solute using freezing point depression. The solvent used will be paradichlorobenzene (PDB), more commonly used as one form of mothballs. PDB has a convenient melting point and a relatively large value for K f , 7.10 ° C/m. The freezing points will be obtained by studying the rate at which liquid PDB and some of its solutions containing the unknown cool in air. Graphs of

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General Chemistry I (FC, 09 - 10) Lab # 13 Molecular Weight Determination by Freezing Point Depression 2 Revised 8/19/2009 temperature versus time, called cooling curves, reveal the freezing points very well, since the rate at which a liquid cools is typically quite different from that of a liquid-solid equilibrium mixture. For an example of a cooling curve, see Graph 1.
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## This note was uploaded on 12/09/2011 for the course CHEM 1 taught by Professor Staff during the Summer '11 term at Simon Fraser.

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Molecular_Weight_by_Freezing_Point_Depression - General...

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