experiment_01 - Experiment 1: Colligative Properties...

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12 Experiment 1: Colligative Properties Determination of the Molar Mass of a Compound by Freezing Point Depression. Objective: The objective of this experiment is to determine the molar mass of an unknown solute by measuring the freezing point depression of a solution of this solute in a solvent as compared to the freezing point of the pure solvent. Background: Colligative properties are properties of a solvent, such as freezing point depression and boiling point elevation, which depend on the concentration of solute particles dissolved in the solvent. The decrease in freezing point, Δ T f (freezing point depression) for a near ideal solution can be described by the equation: Δ T f = k f · m Eq 1 where k f is the molal freezing point depression constant of the solvent with units ° C · kg solvent/mole solute. m is the molal concentration of the solute dissolved in the solvent expressed as moles of solute/kg solvent. Since the molar mass M (traditionally and often, but erroneously called the molecular weight) of a compound has units g/mole, we can solve for moles and substitute the result into the molal concentration relationship, and then into Eq 1 as is shown below. M = g/ mole Eq 2 Rearranging Eq 2 gives moles = g/ M Eq 3 Now substituting Eq 3 into the unit definition of molality yields m = g/( M · kg solvent) Eq 4 And substituting Eq 4 into Eq 1 gives Δ T f = (k f · g)/( M · kg solvent) Eq 5 We can rearrange Eq 5 and solve for the molar mass, mol wt, as is shown in Eq 6, below. M = (k f · g)/( Δ T f · kg solvent) Eq 6 Therefore, if we know the mass of unknown compound added to a known mass of solvent and determine the change in freezing point of the solution, relative to pure solvent, we can use Eq 6 to determine the molar mass of the unknown compound. At the freezing point of any substance, an equilibrium exists in which both liquid and solid are present. liquid solid Eq 7
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13 The temperature at which this equilibrium exists is the freezing point of the substance. Sometimes this temperature is difficult to determine, so the use of cooling curves is required. To construct a cooling curve one would warm their sample, pure solvent or solution, to well above its melting point, then allow it to cool. As the sample cools the temperature of the sample is monitored as a function of time. As the sample begins to solidify the change in temperature will slow, and at the equilibrium shown by Eq 7 the temperature will be constant until all of the sample has solidified. A graph is made by plotting the temperature vs. time. An example of a cooling curve is shown below in Figure 1.
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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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experiment_01 - Experiment 1: Colligative Properties...

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