Background Information
The temperature at which a solution freezes will be lower than the temperature at which the pure solvent freezes.  The difference between the freezing point of a solvent (T°_f) and the freezing point of the solution (T_f) is referred to as the freezing point depression (DT).  A solution’s freezing point is related to the molality (m) of the solution.  The equation that describes the relationship between freezing point depression and molality is:

    DT_f = (K_f)(m)
            DT represents the freezing point depression: DT = T_f°- T_f
                T_f° is the freezing point of the pure solvent.
                T_f is the freezing point of the solution.
                K_f is the molal freezing point depression constant.
                The value of K_f is characteristic for a given solvent.  The units of K_f are given in °C/m.
            m is the molality of the solution. m = moles of solute/ kg of solvent.
 

Example  A solution made from 0.840 g of a substance dissolved in 25.0 g of camphor froze at 170.80oC.  The freezing point of pure camphor is 178.40oC and the molal freezing point depression constant, Kf , for camphor is 37.7°C/m.  Calculate the molecular weight of the solute. DT = Tf°- Tf = 178.40oC – 170.80oC = 7.60oC DTf  =  (Kf)(m) = [(Kf)(g solute)] / [(MW)(kg solvent)] 7.60oC = [(37.7oC/m )( 0.840 g solute)] / [(MW)(0.025 kg solvent)] MW = 167 g/mol

In this experiment, the molecular weight of sucrose (C₁₂H₂₂O₁₁) will be determined from measurements made on the freezing point of pure water (T_f°), the freezing point of an aqueous solution of sucrose (T_f), and the weight of sucrose and water in the solution.

The freezing point of pure water is determined by cooling a sample of water and measuring its temperature as a function of time.  The resulting data are used to construct a cooling curve.  A cooling curve is a plot of temperature (y-axis) vs. time (x-axis) (See Figure 1.).  The freezing point of the solution is determined in the same way as the freezing point of the solvent.  The cooling curve, however, has a different appearance and is shown in Figure 1.
 
 
 
 
 
 
 
 
 
 
 

Figure 1.

Experimental Procedures

Part A.  Preparation of an Ice Bath
Fill a 600 mL or 800 mL beaker approximately ¾ full with ice and cover the ice with 3-4 cm of rock salt.  Stir this ice-salt mixture with a scoopula and make sure the temperature drops to -5 °C to -10 °C.  This ice bath will be used in each experiment.  Be sure to check the temperature of the ice bath before each experiment.  If the temperature has risen above -10°C, then drain the water and add more ice and salt.

Part B.  Determination of the Freezing Point of Water
1.    Add about 25 mL of deionized water to a 25 x 200 mm test tube.

2.    Insert a wire stirrer and thermometer into the test tube.  NOTE: A two-hole stopper is used to hold the stirrer and thermometer.  The thermometer should be inserted into the stopper hole that is split.  The bulb of the thermometer must be below the liquid level inside the test tube.

3.    Place the test tube into the ice bath and clamp the test tube to a ring stand so that the liquid level in the test tube is below the ice.

4.    Stir the liquid inside the test tube continuously at a uniform rate with the wire stirrer and take temperature and time readings at 30-second intervals.  Record the thermometer reading to within 0.5^oC.

5.    Continue taking measurements until 6 successive temperature readings are equal to one another.  All of the liquid inside the test tube may not freeze solid.  Immediately remove the test tube from the ice bath once the readings are complete.

6.    Remove the test tube and allow the water to melt and then repeat the freezing of the water (steps 3-5) for a second data set.  Be sure to check the temperature of the ice bath before performing the second trial.  Make sure that the thermometer is clean before inserting back into the liquid in the test tube.

Part C.  Determination of the Freezing Point of an Aqueous Sucrose Solution
1.    Using a clean dry 25 x 200 mm test tube, prepare a solution of sucrose (C₁₂H₂₂O₁₁) and water by dissolving about 11 g of sucrose in about 25 g (25 mL) of water.  NOTE: Precise measurements of the mass of the sucrose and mass of water are essential.  Record the mass of each to +0.01 g.

2.    Measure the freezing point of the sucrose solution using the procedures from steps 3-5 in part B.

3.    Discard the sucrose solution down the drain, obtain a clean dry 25 x 200 mm test tube and repeat steps 1 and 2 for a second data set.  Be sure to check the temperature of the ice bath before performing the second trial.  Make sure that the thermometer is clean before inserting back into the liquid in the test tube.

Part D.  Calculations
1.    Plot graphs of temperature (y-axis) vs. time (x-axis) for the data sets from each of the trials in Part B and Part C.

2.    Determine the respective freezing point from each graph.

3.    Calculate an average value of the freezing point of the pure water from the graphed results of the two trials in Part B.

4.    Calculate experimental molecular weight values for sucrose from the freezing point depressions of the two trials in Part C.
       NOTE: The molal freezing point depression constant, K_f , for water is 1.86°C/m.

5.    Calculate a percent error for each of the molecular weight values.
 
 

Name________________________________________Hood No.______ Date_____________

Part B. Determination of the Freezing Point of Water

Freezing point of water from graph

Trial #1 __________oC

Trial #2 __________oC

Average _________oC

 

Part C. Determination of the Freezing Point of an Aqueous Sucrose Solution
 

	Trial #1	Trial #2
Mass of sucrose (C12H22O11)	__________g	__________g
Mass of water (g)	__________g	___________g
Mass of water (kg)	___________kg	___________kg
Freezing point of solution from graph	__________oC	___________oC
Freezing point depression	__________oC	___________oC
Experimental molecular weight from freezing point depression data	__________g/mol	__________g/mol
Percent error	________%	________%