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CHEM 01LB Lecture Notes - Lecture 6: Freezing-Point Depression, Melting Point, Colligative Properties

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9 Jun 2018

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CHEM 01LB Experiment 6: Determination of Molar Mass by Freezing Point Depression

●Experimental purpose

○The purpose of this experiment is to determine the molar mass of an unknown

substance by dissolving it in a solvent with a known freezing point constant and

measuring the change in freezing point

●Freezing point depression

○Vapor pressure of a solvent is lowered when a nonvolatile substance is dissolved

in a solvent

■Freezing point is lowered



, and boiling point is raised

●Magnitude of changes depends on number of solute particles

○Colligative properties: properties of a solvent that depend only on the number of

solute particles dissolved

○Relationship between molar mass of an unknown non-dissociating solute and the

freezing point depression of a solvent:

■ΔTf = Kf m

●ΔTf = (freezing point of pure solvent) - (freezing point solution)

●Kf = freezing point constant for solvent

○Freezing point constant for lauric acid: 3.9℃/m

●m = molality



of the solute in the solvent

○Molality = kg solvent

moles solute

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Document Summary

Chem 01lb experiment 6: determination of molar mass by freezing point depression. The purpose of this experiment is to determine the molar mass of an unknown substance by dissolving it in a solvent with a known freezing point constant and measuring the change in freezing point. Vapor pressure of a solvent is lowered when a nonvolatile substance is dissolved in a solvent. Freezing point is lowered , and boiling point is raised. Magnitude of changes depends on number of solute particles. Colligative properties : properties of a solvent that depend only on the number of solute particles dissolved. Relationship between molar mass of an unknown non-dissociating solute and the freezing point depression of a solvent: T f = (freezing point of pure solvent) - (freezing point solution) K f = freezing point constant for solvent. Freezing point constant for lauric acid: 3. 9 /m. M = molality of the solute in the solvent.

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Related Questions

DETERMINATION oF MOLAR EXPERIMENT 23 MASS FROM FREEZING POINT DEPRESSION EQUIPMENT AND CHEMICALS: Equipment 600 mL beaker Chemicals Cyclohexane (C Stir plate 100 ml. graduated cylinder Clamp Nonvolatile Unknowns Freezing point apparatus Spin vane 0.1 C Thermometer INTRODUCTION: When a solute is introduced into a pure solvent the newly formed solution will have certain physical properties that are from the solvent. As a of the presence of the the pressure of the solution will be lower than that of the pure solvent. As the vapor pressure is lowered there is a resulting increase in the amount of energy required to reach the boiling point of the solution. Conversely, the solution must cool to a lower temperature before a state change from liquid to solid occurs. These changes in physical properties vapor pressure lowering, boiling point elevation and freezing point depression are collectively referred to as colligative properties. These colligative properties, which also include osmotic pressure, do not depend on the nature of the nonvolatile solute in the solution but rather on the concentration of solute present. Because the properties are independent of the nature of the solute, they may be used to determine other properties of the solute such as molecular weight. The effect of a nonvolatile, nonelectrolyte solute on a pure solvent will be examined by finding the freezing point depression of a cyclohexane unknown so solution. The freezing point of a pure sample of cyclohexane will be determined. Introducing a nonelectrolyte solute should in solution formed to freeze at a lower temperature. This temperature will then be experimentally determined As stated the The molality of the solution dependent on the quantity of solute particles present in solution. (moles solute particles per kilogram of solvent) is directly proportional to the change in temperature between the freezing point of the pure solvent and that of the ATfss Kim The molal freezing point depression constant, K for cyclohexane is equal to 20.00 cfmolal By experimentally determining the freezing point depression of a ohexanel unknown solution, the molal concentration is easily calculated. In turn the molar mass of the unknown olute can be calculated as well Chemistry 132 EXPERIMENT23

Can someone please help me with this lab. I am so confused as to how to solve for some of these things. Also I'm not sure at all how to answer the "thought" questions. If someone could please help me with these I would greatly appreciate it.