Freezing Point Depression and Boiling Point Elevation Lab

Purpose: In this experiment you will investigate the effect that adding a solute has on the boiling and freezing points of a solvent.

 

Background: When a solute is dissolved in a solvent, the properties of the solvent are changed by the presence of the solute. The magnitude of the change generally is proportional to the number of moles of solute added per kilogram of solvent (molality). Colligative properties of a solution are those properties of a solvent that are changed only by the number of solute particles present, without regard to the particular nature of the solute. These properties include changes in vapor pressure, the boiling point, and the freezing point. When a solute is added to a solvent, the amount of solvent that can escape from the surface of the solution at a given temperature is lowered, relative to the amount of particles that can escape from the pure solvent. Therefore, the vapor pressure above such a solution will be lower than the vapor pressure above of the pure solvent under the same conditions. Molecules of the solute physically block the surface of the solvent, thereby preventing as many solvent molecules from evaporating at a given temperature. The presence of a solute lowers the temperature at which the solution freezes and raises the temperature at which the solution boils relative to the pure solvent.

 

Materials:      100ml Graduated Cylinder                    Thermometer                Ring Stand and Ring
150ml Beaker                                       Stirring Rod                  Clamp
250ml Beaker                                       Wire Gauze                  Bunsen Burner
Crushed Ice                                          Distilled Water  Sugar (Sucrose)
Salt (Sodium Chloride)             Calcium Chloride          Slotted Rubber Stopper

 

Procedure:

  1. Freezing Point Depression

·        Because of the difficulties that would be involved in trying to weigh a sample of ice accurately, we will be making only a “semi-quantitative” study of freezing point lowering.

·        Weigh out separately the following samples for use in the freezing point study: 34.2g of sucrose, 5.84g of sodium chloride, and 11.0g of calcium chloride. Each of these samples represents 0.1mol of the respective substance.

·        Fill the 150mL beaker with ~100ml crushed ice, packing the ice as tightly as possible in the beaker. Add 25.0mL of distilled water to each beaker. By adding the liquid water, we are constructing the solid-liquid equilibrium system, whose temperature should be exactly 0˚C as long as any ice is present in the system. Although we do not know the exact mass of water (both ice and liquid) in the beaker, we can assume that the total amount in beaker is very nearly 100ml.

·        Determine the temperature of the ice/water mixture. Make certain that the thermometer is held in the middle of the ice/water mixture and does not touch the walls or bottom of the beaker. Note if the temperature as read on your thermometer differs from 0˚C. Keep this error in mind when reading the temperatures of the experimental solutions during the rest of this experiment.

·        Add the weighed sample of sucrose to the 150mL beaker containing the ice/water mixture. Stir the sugar with a stirring rod until it dissolves as much as possible. Then determine the temperature of the ice/water/sugar mixture. Record the value after making any adjustments based on error noted when measuring the temperature of the ice/water mixture prior to adding the solute.

·        Repeat the process using fresh ice/water with the sodium chloride and then again with the calcium chloride sample in place of the sucrose. Record the freezing point temperature of the mixture after making any adjustments based on error noted when measuring the temperature of the ice/water mixture prior to adding the solute.

 

  1. Boiling Point Elevation

 

Data Table: Aqueous Solutions

Freezing Points

Particles

Compound

Mass Solute

Measured

Temp. ˚C

Calculated

Temp. ˚C

Error

Sucrose

C12H22O11

1mol/1mol

doesn’t ionize

34.2g

 

 

 

Sodium Chloride

NaCl

2 moles ions

1 mole comp

5.84g

 

 

 

Calcium Chloride

CaCl2

3 mole ions

1 mole comp

11.0g

 

 

 

Boiling Points

 

 

 

 

 

Sucrose

C12H22O11

1mol/1mol

doesn’t ionize

34.2g

 

 

 

Sodium Chloride

NaCl

2 moles ions

1 mole comp

5.84g

 

 

 

Calcium Chloride

CaCl2

3 moles ions

1 mole comp

11.0g

 

 

 

 

Calculations: Molality = m = moles of solute particles / kg of solvent
Tf = Kf m                    Kf = -1.86˚C/molal                   FP = 0˚C + ∆Tf
∆Tb = Kb m                  Kb= 0.52˚C/molal                     BP = 100˚C + ∆Tb

1.      Convert the mass of each solute into moles.

2.      Calculate the number of moles of particles (ions) for each solute using particles/compound in the above table.

3.      Calculate the molality for each of the three solutions assuming that each solute was dissolved in 100ml of water. (Density of H2O = 1.0g/ml.)

4.      Calculate the ∆T for the FP of each solution and add it to the FP of H2O. Record this value in the data table.

5.      Calculate the ∆T for the BP of each solution and add it to the BP of H2O. Record this value in the data table.

6.      Calculate the difference between the measured value from the lab and the calculated value from the formula. Record this error in the data table.

Conclusion:

1.      Which solute produced the largest depression in the FP? Which solute produced the largest elevation in the BP?

2.      Which solute produced the smallest depression in the FP? Which solute produced the smallest elevation in the BP?

3.      Are the measured temperatures for the solutions consistent with the trend you would expect based on the number of particles produced when each of the respective solutes dissolve? EXPLAIN.

4.      Explain the differences between the measured temperatures and the calculated temperatures (i.e. list three possible sources of error).

5.      Describe, on a microscopic basis, how a liquid boils and how the addition of the solute changes this process.

6.      A solution is made consisting of 2.05g aluminum chloride (AlCl3) dissolved in 9.87g of water (H2O). Calculate the FP and the BP of this solution.