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Freezing Point Depression Lab
Chemistry Materials from Ray Tedder

Below is a web version of the lab handout that students will receive.

Freezing Point Depression Lab

 

The Lab Problem: Determine how much a 1.0 molal salt solution lowers the freezing point of water.

 

Introduction

Have you ever wondered how antifreeze lowers the freezing point of water in the cooling systems of automobiles? In the late nineteenth century the French chemist Francois Raoult noted that the vapor pressure of a solvent was lowered by the addition of a solute (Raoult's Law), and that the freezing points of solutions also were lowered.

 

Eventually, the following mathematical expression that related freezing point depression and molality (moles of solute per kilogram of solvent) was developed.

 

DT = Kf msolute i

 

In this equation, DT is the freezing point depression, Kf is the molal freezing point constant for the solvent, and m is the molality of the particles in the solution.

 

As you can see from the equation, the lowering of the freezing point depends on the concentration of dissolved particles present. In the case of a non-electrolyte, like antifreeze, the molality of the non-electrolyte and the molality of particles in solution are the same (a 1:1 ratio). For electrolytes, like NaCl, the molality of particles is equal to the molality of the electrolyte times the number of ions in the chemical formula of the compound. The correction factor (i) is needed due to the attraction between solvent particles.  This correction factor is called the van't Hoff Factor.   The accepted van't Hoff Factor (i) for NaCl is 1.9.

 

Procedures:

 

1.      Using a marker, label a test tube and the 250mL beaker "D" for distilled water. Then label a test tube and a plastic cup "N" for sodium chloride.

 

2.      Fill the 250mL beaker labeled "D" with 200mL of distilled water.

 

3.      Using a pipet, transfer the distilled water into the test tube labeled "D" to a height of 3cm.

 

4.      Mass the plastic cup labeled "N". Record its mass.

 

5.      Pipette 50.0mL of distilled water into the plastic cup.

 

6.      Mass the plastic cup with the water. Record its mass.

 

7.      Measure exactly 2.92 grams of sodium chloride. Record this mass.

 

8.      Add the sodium chloride to the plastic cup labeled "N" containing 50.0mL of water.

 

9.      Using a stirring rod, mix the solution until all of the salt has dissolved.

 

10. Using a pipet, transfer the sodium chloride solution into the test tube labeled "N" to a height of 3cm.

 

11. Fill the 400mL beaker with ice to the 300mL mark. Cover the ice with a dusting of sodium chloride crystals. Stir this ice-salt mixture with a stirring rod until it reaches a constant temperature at or below -10oC.

 

12. Hold the test tube containing distilled water with a test tube holder, and place it into the ice-salt bath. Using the thermometer, gently stir the water until ice crystals first appear. Hold the thermometer at the very top of the water. Record this temperature.

 

13. Hold the test tube containing sodium chloride solution with a test tube holder, and place it into the ice-salt bath. Using the thermometer, gently stir the water until ice crystals first appear. Hold the thermometer at the very top of the solution. Record this temperature.

 

Calculations:

 

1.      Calculate the mass of the water, in kilograms.

fpdcalculation1.jpg

 

2.      Calculate the number of moles of NaCl used.

fpdcalculation2.jpg
 
3.      Calculate the molality of the NaCl solution.  
fpdcalculation3.jpg

4.      Calculate the experimental DT for the NaCl solution.

      Temp (in C) at which pure distilled water started freezing

        minus temp (in C) at which the 1.0 molal salt solution started freezing =

 

5.      The accepted K f for water is 1.86 oC/m (remember that m is molality). 

      The accepted i for NaCl is 1.9. Use the equation from the introduction of this lab to calculate the accepted DT for the solution.

 

6.      Calculate the % of error for your experiment.    

fpdcalculation4.jpg

Note: As part of this lab, we will be making homemade ice cream.  To eat ice cream, you will have pay for your share of the supplies at least 1 week in advance.  Students will also have to bring ice cream freezers to make ice cream.  Regardless of whether you eat ice cream, you will have to use the knowledge gained in this lab to explain how adding ice to the salt in the homemade ice cream freezer was necessary to get the ice cream to freeze.

If any of this confuses you, feel free to contact the class instructor for help.

Click here to email Mr. Tedder.