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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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