Forces and Bonding: Colligative Properties

1 lab period; work in pairs. Complete the Preparation page before laboratory.

• To observe the relative vapor pressures of several liquids
• To observe the vapor pressure lowering caused by a solute
• To observe the osmotic pressure phenomenon
• To observe the freezing point depression
• To observe the boiling point elevation

When a non-volatile solute is dissolved in a liquid solvent, certain physical properties of the liquid solvent are quantitatively altered. Interestingly, the magnitude of the alteration is directly proportional to the amount of solute dissolved, but does not depend at all on the identity of the solute (i.e., on what the solute molecules are like). These properties are collectively called colligative properties. and are summarized below.

1. Vapor pressure lowering. The presence of a non-volatile solute causes a decrease in the vapor pressure of the solvent. The magnitude of the decrease is directly proportional to the concentration of solute, expressed as the mole fraction:
   
   DP_vap = X_solute P^o_vap where P^o_vap is the vapor pressure of the pure solvent.
2. Boiling Point Elevation. The presence of a non-volatile solute causes the solvent to boil at a higher temperature. The magnitude of the so-called boiling point elevation is directly proportional to the concentration of solute, expressed as the molality, m:
   
   DT_b = K_b*m K_b is called the boiling point elevation constant of the solvent.
3. Freezing Point Depression. Similarly, a non-volatile solute causes the solvent to freeze at a lower temperature than normal. The magnitude of the freezing point depression is directly proportional to the molality of solute:
   
   DT_f = K_f*m K_f is called the freezing point depression constant of the solvent.
4. Osmotic Pressure. Molecules of solute spread uniformly throughout and moving within a definite volume of solvent are analogous in a number of ways to gas molecules in a container. Just as the gas molecules exert pressure on the walls of the container, solute molecules exert a "pressure" when confined within the solvent volume. If the solution is placed in contact with pure solvent via a particular type of membrane, solvent molecules will pass through the membrane from the pure solvent to the solution, resulting in dilution (and expansion) of the solution. This solution expansion is mathematically analogous to the expansion of a gas against a lower outside pressure.
   The pressure exerted by the molecules of solute confined in a particular volume of solvent is called the osmotic pressure. It is given the symbol P and obeys an ideal gas type law:
   
   PV = nRT Here V is the volume of solution, and n is the number of moles of solute dissolved in that volume of solution. R is, of course, the ideal gas constant.

In this experiment, you will carry out a number of simple experiments in order to observe the four colligative properties. As usual, careful observation and careful work are necessary if you are to obtain reliable results.

Although space does not permit a discussion of the importance of colligative properties, these are indeed very important phenomena. Particularly in biology, cells must maintain a delicate balance with their surrounding medium in order not to be destroyed by osmotic pressure, either by explosion or implosion.

Focus Questions

Boiling Point Elevation

Vapor Pressure

Freezing Point Depression

Osmotic Pressure

Overall

Equipment and Materials

• Assorted beakers (50-100 mL) and Erlenmeyer flasks (25, 50-mL)
• Pasteur pipets
• 5-mL syringe
• osmometer (glass cylinder with open ends; one-hole rubber stopper with capillary inserted; rubber sleeve; semipermeable membrane)
• Two 5-mL graduated pipets
• 6-inch length of tygon tubing
• 1.5-foot length of vacuum tubing
• 1-mL syringe with needle
• sawed-off 1-mL syringe with needle
• syringe needle
• sidearm test tube
• 10-mL glass syringe
• Septum caps to fit Erlenmeyer flasks
• large beaker
• rock salt
• ice
• 25- or 50-mL roundbottom flask
• Water condenser
• Wide-stem plastic funnel
• Liquids: water, acetone, dichloromethane, carbon tetrachloride, toluene, ethanol, methanol
• solutes: benzophenone, sucrose, NaCl, bovine serum albumen

Safety glasses must be worn at all times in the laboratory. You will use mercury thermometers. Mercury is an extremely toxic substance. Report thermometer breakage IMMEDIATELY to your instructor, who will take the necessary steps to clean up the mercury spill.

Experimental

Record all data in your notebook.

Osmotic Pressure. Prepare a solution of 0.10 g of bovine serum albumen (molar mass ~ 68000 g/mole) in 5 mL of (distilled) water. Set aside to allow the protein to dissolve. Attach the semipermeable membrane to the glass tube of the osmometer as shown by the instructor. Fill the tube with albumen solution, and insert the rubber stopper/capillary in the open end of the glass tube. Tightly wrap the stopper/tube interface with parafilm. Place the assembly in a beaker of water so that the level of solution in the capillary matches the level of water in the beaker. Cover the beaker with aluminum foil. Label the beaker with your name(s) and place it in a location where it will not be disturbed.

After 1 week, observe the level of liquid in the capillary.

Boiling Point Elevation. Measure 25 mL of water into a small round bottom flask and clamp the flask to a ring stand over a ring/wire gauze and Bunsen burner. Insert a condenser into the mouth of the flask. Connect the bottom condenser tube to a water faucet. Cut a 6-inch length of fine steel wire and pass the wire throught the eyehole at the top of the thermometer. Twist the 2 ends of the wire together to form a loop, and use the loop to lower the thermometer through the condenser and into the water. For the time being, the thermometer may rest against the bottom of the flask. Turn on the flow of water to the condenser, and heat the water in the flask gently to boiling with a bunsen burner. When the water is boiling steadily, reduce the flame enough to just maintain a steady boil and measure the temperature of the water to the nearest 0.1 ^oC. When you make this measurement, be sure that the mercury bulb of the thermometer is immersed in the water in the round bottom flask, but does not touch the bottom of the flask. Make sure that the boiling temperature of the liquid remains constant.

Turn off the burner flame and allow the water to cool for 10 minutes. Then add 1.5 g of NaCl to the water in the round bottom flask, and bring the solution to a steady boil. Again measure the boiling temperature of the liquid.

Again cool for 10 minutes, then add a further 1.5 g of NaCl and determine the boiling point of the solution.

Vapor Pressure.

Method 1: Evaporation Rate of Liquid Drops. Into labelled 1-dram vials, transfer 0.5 mL each of water, acetone, and ethanol, respectively, and cap the vials.

To measure relative vapor pressure, we will determine the amount of time required for the same number of molecules of each liquid to evaporate. Before carrying out the experiment, it is necessary for you to determine what volumes of ethanol, dichloromethane, and acetone contain the same number of molecules as are contained in 1 microliter of water. The following data will enable you to make the calculations.

Rinse a 5-mL syringe three times with one of the 3 liquids, then draw up exactly the volume calculated above. Eject the syringe contents into the well of a well plate, and measure the time required for the liquid to evaporate. Your goal here is to achieve reproducibility in technique, so that the precision in measured time is no worse than about 5 percent (1 second in 20 seconds total). Make repeated ejections of the first liquid until you are confident that you are ejecting and measuring the time exactly the same way every time.

Rinse the syringe with a second liquid and measure the time required for the calculated volume to evaporate completely. Note: WATER AND DICHLOROMETHANE ARE IMMISCIBLE SO CANNOT BE DONE IN SUCCESSION, BECAUSE ONE WILL NOT RINSE THE OTHER OUT OF THE SYRINGE. Make at least 5 measurements and determine the average time.

Determine the evaporation time for the third liquid, and then the fourth.

Method 2: Gas Chromatographic Determination of Vapor Pressure. In a 1-dram vial, prepare 1 mL of a solution of benzil (C₁₄H₁₀O₂) in acetone (C₃H₆O) having mole fraction benzil = 0.2. In a second 1-dram vial, prepare 1 mL of a second solution having mole fraction benzil = 0.1 by appropriate dilution of a portion of the first solution. Using a 2-mL gas-tight syringe, sample 0.4 mL of the vapor phase from the vial containing pure acetone, dilute it with 0.6 mL of air, and inject the mix to a gas chromatograph. Repeat 4 more times, then determine the signal areas. Make similar injection series from the vials containing the two benzil solutions, and determine signal areas.

Method 3: Manometric Determination of Vapor Pressure. Construct a manometer by connecting the tip-ends of two 5-mL graduated pipets with a 6-inch length of tygon tubing. The tubing should fit snugly onto the ends of the pipets. To be sure of a leak-free fit, wrap the connections with strips of parafilm. With a rubber band, fasten the two pipets side-by-side with the marks aligned, then clamp the manometer to a ring stand. Fill the manometer with distilled water until both pipets are filled to the 1.5-mL mark.

Attach a length of rubber vacuum tubing to one side of the manometer. Make sure that the fit of tubing to glass is tight! Wrap with a strip of parafilm. Into the other end of the rubber tubing, insert a "sawed-off" syringe barrel with attached needle and needle cap. Make sure the needle is firmly attached to the syringe barrel, then wrap both the tubing/syringe and syringe/needle connections with strips of parafilm.

In the hood, pour 1-2 mL of CCl₄ into a 1-dram vial, and cap the vial. Take it to your work area and place it on a labelled paper towel strip.

Fit a clean, dry 25-mL Erlenmeyer flask with a new septum cap. Be sure the fit is tight. Fold the septum flap down over the mouth of the flask. Then insert the manometer needle thru the center of the septum cap. Is there any response at the manometer? Vent the flask by inserting a small syringe needle until the liquid is at the same level in both manometer arms. Then remove the vent needle from the flask.

Leak testing. Using a small syringe, inject a 2-mL shot of air into the manometer flask and monitor the manometer response for 1 minute. The liquid levels in the arms of the manometer should register the increase in inside pressure, and should remain at constant height for at least 1 minute following air injection. If the liquid levels slowly move toward equalization, there is a leak in your system. This leak must be found and eliminated before you proceed, because it will prevent you from obtaining reproducible, reliable data.

Vapor Pressure of Pure Liquid. Using a small syringe, draw up about 0.1 mL of CCl₄. Insert the syringe needle through the septum cap of your manometer flask, and inject the liquid to the flask. Watch the manometer response. When the equilibrium vapor pressure has been established, use a ruler to measure the distance separating the water levels in the manometer arms. Then remove the manometer needle from the flask. Using your sink aspirator, remove CCl₄ vapor from the needle side of your manometer by slowly and carefully inserting the needle into the end of the aspirator tube. Be sure to do this carefully to avoid sucking all of the water out of your manometer! Remove the septum cap from the flask and dump the contents into a bottle for halogenated organic waste. Then place the flask on your sink aspirator to dry. Using a second clean/dry flask, repeat the procedure for measurement of vapor pressure of CCl₄ until you obtain a consistent reproducible value. You will probably need to make a minimum of 4 runs to accomplish this.

Transfer 1.00 mL of CCl₄ to another 1-dram vial. Add 0.44 g benzophenone, cap, and swirl or shake to dissolve the solute. Then measure the vapor pressure of the solution using exactly the same procedure as used above. Repeat to obtain a reliable value for the vapor pressure of the solution. Transfer the contents of flasks to the halogenated organic waste bottle. Rinse flasks with acetone, and pour this in a nonhalogenated organic waste bottle.

Method 4: Syringe Method for Determination of Vapor Pressure. Obtain a small side-arm test tube, a 10-mL glass syringe, a septum cap to fit the mouth of the test tube, and a 1-mL plastic syringe. The sidearm test tube and syringe must be absolutely CLEAN and DRY. Please make sure of this before you start. Once the equipment is clean and dry, remove the glass syringe plunger and lay it on the bench top while you do the following operations. Attach a short length of appropriately sized rubber tubing to the end of the glass syringe. The fit must be tight. Attach the other end of the tubing to the sidearm of the test tube. This fit must also be tight. Use ring stands and clamps to mount the test tube vertically and the syringe horizontally. BE SURE THAT THE SYRINGE IS EXACTLY HORIZONTAL SO THAT WHEN THE PLUNGER IS REPLACED, IT WILL NOT FALL OUT AND BREAK. Then replace the plunger in the barrel of the syringe. Fit the solid end of the septum cap tightly into the top of the test tube and fold the hollow end over the lip of the tube. At this point, the setup should look as shown. Insertion of the septum cap will probably affect the position of the syringe plunger. Write down the plunger position.

In the hood, pour 1-2 mL of CH₃OH (methanol) into a 1-dram vial, and cap the vial. Take it to your work area and place it on a labelled paper towel strip.

Now use the small plastic syringe to inject about 0.1 mL of CH₃OH into the sidearm test tube. To do this, you will need to puncture the center of the septum cap with the syringe needle. Once the liquid is in, remove the small syringe from the septum cap, and observe the position of the plunger of the glass syringe. Write down the final plunger position. Remove the septum cap from the test tube, and aspirate the test tube to remove ALL TRACES of CH₃OH, both liquid and vapor.

Repeat the measurement of the vapor pressure of pure liquid until you are confident of the reproducibility of the value to within 10%.

Transfer 1.00 mL of CH₃OH to another 1-dram vial. Add 0.10 g benzophenone, cap, and swirl or shake to dissolve the solute. This may take a few minutes. Then measure the vapor pressure of the solution using exactly the same procedure as used above. Clean the sidearm test tube by rinsing several times with small portions of acetone, then aspirate dry. Repeat the experiment to obtain a reliable value for the vapor pressure of the solution. Transfer the contents of the vials to the nonhalogenated organic waste bottle. Rinse vials with acetone, and pour this in a nonhalogenated organic waste bottle.

Measure the volume capacity of the side arm test tube by inserting the septum cap, and filling the tube through the sidearm with distilled water. When the tube is completely full, pour the contents into a graduated cylinder. The volume of water in the cylinder provides a good estimate of the volume capacity of the sidearm test-tube portion of the vapor pressure assembly.

Use your data to calculate the vapor pressures of pure CH₃OH and the solution of benzophenone in CH₃OH, in torr, using the following equation:

Be sure to thoroughly clean and dry the sidearm test tube and syringe (if necessary) before proceeding to the next part of the experiment.

Freezing Point Depression. In a 250-mL beaker, prepare an ice-salt bath as follows. Fill the beaker halfway with ice. Add half this volume of rock salt, then add 50 mL of cold water. Stir the mixture vigorously with your glass stirring rod, periodically measuring the temperature of the slush. You will occasionally need to add ice to the slush to maintain the bath temperature.

Quickly measure 20 mL of water into a 25-mL Erlenmeyer flask, and clamp the flask to that its contents are submerged in the cold bath. Suspend or lightly clamp a thermometer so that its bulb is submerged in the water in the Erlenmeyer. While stirring frequently, monitor the temperature of the water as a function of time until the water begins to freeze, then for at least 2 minutes after the onset of freezing.

Remove the Erlenmeyer from the bath, pour out the water, and replace it with 20 mL of room temperature water. Add 1.2 g of NaCl to the Erlenmeyer and swirl to dissolve. Refresh the bath if necessary, then again mount the Erlenmeyer in the bath. Monitor the temperature of the water in the Erlenmeyer as a function of time until freezing begins, and for at least an additional 2 minutes.

Warm the solution in the Erlenmeyer, add an additional 1.2 g of NaCl, and again take the cooling curve.

Clean-up. When you have finished all of your work:

• Clean all glassware with Alconox and water and shake dry.
• Rinse Pasteur pipets by inserting the nozzle of your wash bottle in the top end and squirting water through. Aspirate dry.
• Return all components to the labkit.
• Return all borrowed equipment, including 1-dram vials, to the instructor before leaving lab.

All halogentated organic solvents (carbon tetrachloride, dichloromethane) should be disposed of in a halogenated organic waste bottle. All non-halide containing organic solvents should be disposed of in a nonhalogenated organic waste bottle. Aqueous NaCl solutions may be disposed of in the sink.

Preparation Questions