Experiment 6: Determination Of Molar Mass By Freezing Point Depression Flashcards
What is the purpose of this lab?
The molar mass of an unknown substance will be determined by dissolving the substance in a solvent with a known freezing point constant and measuring the change in freezing point.
What is nonvolatile substance?
Substances that don’t easily transfer into the gaseous phase.
What happens when a non-volatile substance is dissolved in a solute?
The vapor pressure of the solvent is lowered. As a result, the freezing point of the solution is lowered and the boiling point is raised relative to the pure solvent. The magnitude of these changes depend on the number of solute particles (molecules or ions) present in a given mass of solvent.
Define Colligative Properties
Properties of a solvent that depend only on the number of solute particles dissolved, and are independent of the nature of the solute, are called colligative properties.
What is the important equation for this experiment?
DELTATf= Kf .m
Where DELTATf (freezing point of pure solvent)-(freezing point of solution)
Kf= freezing point constant for the solvent
M= molality of the solute in the solvent.
What is the solvent for this experiment and what is its freezing point constant?
Lauric Acid (C12H24O2); 3.9
What will be observed with the cooling of pure lauric acid solvent?
As cooling begins, the temperature of the lauric acid will decrease rapidly. However, as soon as the solvent begins to freeze, the temperature remains constant until all of the lauric acid has solidified. When that occurs, the temperature again decreases. The freezing point is the constant temperature that occurs while the solvent is freezing.
How can the freezing of a solution with lauric acid as the solute be observed?
Initial cooling results in a rapid decrease of temperature. However, unlike the pure lauric acid solvent, the temperature does not remain constant until all the solvent has frozen. As the lauric acid freezes, the solution becomes more concentrated because less liquid remains. The molal concentration increases; as a result, the freezing point further decreases. The freezing point can be obtained by drawing two solid lines of best fit.
