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42-0149-00-02-EXPColligativePropertiesandOsmosticPressure.pdf

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Colligative Properties and Osmotic Pressure Hands-On Labs, Inc. Version 42-0149-00-02

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

Experiment Summary:

You will explore the colligative properties of freezing point, boiling point, and osmotic pressure in solutions. You will observe changes to these colligative properties by adding a controlled amount of solute to a solution. You will define colligative properties as well as discuss membrane permeability and osmotic pressure.

EXPERIMENT

Learning Objectives Upon completion of this laboratory, you will be able to:

● Explain the four colligative properties of a solution: vapor pressure, freezing point depression, boiling point elevation, and osmotic pressure.

● Describe the process of osmosis and define osmotic pressure.

● Describe the mathematical relationship between osmotic pressure, molarity, and temperature.

● Observe and describe the process of osmosis through a semipermeable membrane.

● Determine the molecular mass of a compound using osmotic pressure data.

● Examine how the freezing and boiling points of solutions change as a result of the amount of solute present.

Time Allocation: 2.5 hours

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Experiment Colligative Properties and Osmotic Pressure

Materials Student Supplied Materials

Quantity Item Description 1 Aluminium pie pan 1 Bottle of distilled water 1 Crushed ice 1 Glass bowl 1 Light corn syrup, 2 oz. 1 Matches or lighter 1 Measuring spoon, 0.5 tsp 1 Plastic drinking cup 1 Roll of paper towels 1 Salt 2 Small rubber bands 1 Source of tap water 1 Timer, clock, or watch with second hand

HOL Supplied Materials

Quantity Item Description 1 Aluminum cup, 2 oz 1 Burner fuel 1 Burner stand 1 Dialysis tubing, 6 in 1 Digital scale 1 Funnel 1 Glass beaker, 250 mL 1 Graduated cylinder, 25 mL 1 Pair of safety gloves 1 Pair of safety goggles 1 Thermometer

Note: To fully and accurately complete all lab exercises, you will need access to:

1. A computer to upload digital camera images.

2. Basic photo editing software such as Microsoft Word® or PowerPoint®, to add labels, leader lines, or text to digital photos.

3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.

Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit.

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Experiment Colligative Properties and Osmotic Pressure

Background Solutions and Vapor Pressure

A solution consists of a solvent (the dissolving medium) and one or more solutes (substances dissolved into the solution). Most of the properties of a solution are dependent on the properties of the solute. However, colligative properties are four properties that are not dependent on the identity of the solute, but dependent on the number (concentration) of solute particles that are present in the solution. The four colligative properties of solutions are:

1. Vapor pressure lowering

2. Boiling point elevation

3. Freezing point depression

4. Osmotic pressure

Consider a liquid that is put into an open container at room temperature, such as water in a beaker. After a period of time, all the liquid will evaporate. Evaporation occurs because the molecules in a liquid escape the surface of the liquid to form a gas, or vaporize due to the pressure of the moving molecules. Any volatile substance, or substance that will evaporate, has a specific vapor pressure. The rate of evaporation is dependent upon environmental conditions such as air temperature, air pressure, and humidity.

Solutions that contain a solvent and non-volatile solute particles have lower vapor pressures than the pure solvents under the same conditions. See Figure 1. The presence of the non-volatile solute particles reduces the likelihood that the solvent will evaporate. Each component of the solution, the non-volatile solute and the solvent, lowers the vapor pressure of the other.

Figure 1. Adding a solute to a solution decreases the vapor pressure.

Boiling point is defined as the temperature at which the vapor pressure of a liquid is equal to the atmospheric pressure on the liquid. As solutions (with non-volatile solutes) have lower vapor pressures than pure solvents at the same temperature, a higher temperature is required to boil the solution than the pure solvent. The elevated boiling points are directly related to both the number (concentration) of solute particles and the nature of the solvent.

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Experiment Colligative Properties and Osmotic Pressure

The presence of solute also interferes with the formation of a solid as the solution cools. As a result, solutions with non-volatile solutes have lower freezing points than pure solvents. Just as for the boiling point, the lowered freezing points are directly related to both the number (concentration) of solute particles present and the nature of the solvent. See Figure 2.

Figure 2. Adding a non-volatile solute to a volatile solvent will decrease the vapor pressure of the solvent. © Volker Rauch

An antifreeze is an additive which

lowers the freezing point of a water- based liquid. An antifreeze mixture is

used to achieve freezing-point depression for cold environments and also achieves boiling-point elevation (“anti-boil”) to

allow higher coolant temperature.

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Experiment Colligative Properties and Osmotic Pressure

Osmosis and Osmotic Pressure

If two solutions of different concentrations are separated by a semi-permeable membrane, solvent from the less concentrated solution will pass through the membrane into the more concentrated solution, diluting it. This process will continue until the concentration of solute is the same on each side of the membrane. This phenomenon is called osmosis.

In biological systems, if a cell is placed into a salt solution in which the salt concentration in the solution is lower than in the cell, the solution is said to be hypotonic. Water will move from the solution into the cell and the cell will expand to the point where it may burst. On the other hand, if a cell is placed into a salt solution in which the salt concentration in the solution is higher than in the cell, the solution is said to be hypertonic. Water will move out of the cell and into the solution, causing the cell to shrink.

The pressure that must be applied to stop the movement of solvent through the membrane is the osmotic pressure of the solution. The amount of pressure needed to stop the movement of solvent is related to the concentration of solute particles in the two solutions. It is important to note that a semipermeable membrane can be specifically selected to allow passage of some small particles. Likewise, biological membranes may allow passage of certain small molecules and not others. See Figure 3.

Figure 3. Osmotic pressure, due to a semipermeable membrane, will cause molecules to move one direction in an attempt to maintain equilibrium on both sides of the membrane. © Dreamy

Girl

Osmosis is the basis for hemodialysis, where

the blood of patients with kidney malfunction is filtered

to remove waste products normally removed by the

kidneys.

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Experiment Colligative Properties and Osmotic Pressure

Colligative properties and molar mass calculations

Historically, certain colligative properties: freezing point depression, boiling point elevation, and osmotic pressure have been used to determine the molar mass of a solute. Now there are instrumental methods to determine this. Of these three, osmotic pressure is the most sensitive and gives the best results. The molar mass of a solute in a solution can be found through the following equation:

Sample Problem:

0.125 grams of a protein were dissolved in 100 mL of water at 25°C. The solution has an osmotic pressure of 5.15 mm Hg. What is the molar mass of the protein?

As the gas constant, R, requires atmospheres as pressure units, we first have to convert 5.15 mm Hg to atmospheres, and the temperature from Celsius to Kelvin:

The values can then be placed into the equation:

Solve for M (Molarity = mol/ L):

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Experiment Colligative Properties and Osmotic Pressure

0.125 grams of a protein were dissolved in 100 mL of water. Therefore, the mass of the protein per liter of solution is:

We now know that the concentration of the solution (2.77 x 10-4 M) was produced by adding 1.25 grams of protein to 1 liter of water. Therefore, 2.77 x 10-4 mol of the protein has a mass of 1.25 grams. The molar mass of the protein is calculated as shown below:

In the following exercises, you will explore three of the four colligative properties of a solution: freezing point, boiling point, and osmosis.

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Experiment Colligative Properties and Osmotic Pressure

Exercise 1: Colligative Properties - Osmosis In this exercise, you will observe the results of osmosis through a semipermeable membrane.

Note: Completely read all instructions and assemble all equipment and supplies before beginning work on this experiment.

Procedure

1. Gather the bowl, distilled water, and the piece of dialysis tubing.

Note: Treat the dialysis tubing gently as you only have 1 piece available.

2. Soak the dialysis tubing for 5 minutes in a bowl filled with distilled water. Ensure the tubing is completely submerged.

3. While the dialysis tubing is soaking, gather the funnel, 2 rubber bands, the light corn syrup, and some paper towels.

4. After the tubing has soaked 5 minutes, remove it from the water and put it on a paper towel.

5. Pour the water down the drain and rinse the bowl with distilled water.

6. Refill the bowl halfway with distilled water.

7. To open the dialysis tubing, carefully rub the dialysis tubing between your fingers until the middle of the tubing opens. See Figure 4.

Figure 4. Carefully rubbing the dialysis tubing until the middle of the tubing opens.

8. Carefully close off 1 end of the dialysis tubing by folding the end and then tying it off using a cut rubber band. See Figure 5.

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Experiment Colligative Properties and Osmotic Pressure

Note: You may want to cut the rubber band and tie off the tubing or you may want to just wrap the rubber band around the tubing several times. Test the closure by adding distilled water to the tube. Repeat the tying process if the closure leaks. Empty the tubing of distilled water after testing.

9. Insert the funnel into the open end of the dialysis tubing and hold the tubing snugly to the funnel during the next step. See Figure 5.

Figure 5. Inserting the funnel into the open end of the dialysis tubing.

10. Use the funnel to carefully fill the dialysis tubing ⅓ full with the light corn syrup. Remember that the corn syrup is viscous, so it will move into the tubing slowly. Try to avoid getting any light corn syrup on the outside of the tubing.

11. Carefully push out most of the air that is inside the tubing above the corn syrup. Then close off the open end with the rubber band (as done previously). If any corn syrup does get onto the tubing, after folding over the other side of the dialysis tubing and tying it off with a rubber band, gently rinse the outside of the tubing with distilled water.

12. Gently pat the tubing dry with a paper towel. Place the plastic cup on the digital scale and tare the scale. Then, place the dialysis tubing with the light corn syrup inside the plastic cup to determine the mass of the dialysis tubing/corn syrup. Record the mass in Data Table 1 of your Lab Report Assistant at time “0.”

13. Place the tubing filled with light corn syrup into the distilled water in the cup or bowl. Ensure that the tubing is completely submerged. Allow the tubing to remain in the water for 30 minutes.

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Experiment Colligative Properties and Osmotic Pressure

14. After 30 minutes, remove the tubing from the water. Gently pat the tubing dry with a paper towel and determine the mass of the dialysis tubing/corn syrup as done previously in Step 12. Record the mass in Data Table 1.

15. Place the tubing filled with light corn syrup back into the distilled water in the cup or bowl. Ensure that the tubing is completely submerged. Allow the tubing to remain in the water for another 30 minutes.

Note: Use the same distilled water in the bowl, DO NOT replace the water.

16. After 30 minutes, remove the tubing from the water. Gently pat the tubing dry with a paper towel and determine the mass of the dialysis tubing/corn syrup as done previously in Step 12. Record the mass in Data Table 1.

Cleanup:

17. Pour the liquids down the drain, wash and rinse the bowl, and place the used dialysis tubing into the garbage.

Questions A. In your experiment, is the light corn syrup in the dialysis tubing hypertonic or hypotonic to

the water?

B. 0.302 grams of an antibiotic was dissolved in 500 mL of water at 23.6°C. The solution has an osmotic pressure of 8.34 mm Hg. What is the molar mass of the antibiotic? Show your work.

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Experiment Colligative Properties and Osmotic Pressure

Exercise 2: Colligative Properties - Freezing Point In this exercise, you will determine the freezing point of tap water and 2 more solutions with varying amounts of salt (a nonvolatile solute) added to them.

Note: Completely read all instructions and assemble all equipment and supplies before beginning work on this experiment.

Procedure

1. Gather the following equipment and set it up near a source of tap water: the large plastic cup, crushed ice (made from tap water), measuring spoon for a 0.5 teaspoon (~2.5 mL), salt, digital scale, thermometer, stopwatch, and 25 mL graduated cylinder.

2. Place 100 g of ice in the plastic cup.

● Place a cup on the digital scale and tare the cup.

● Carefully place enough ice in the cup, by adding small pieces, until you get 100.0 g. See Figure 6.

Note: If the digital scale in your kit measures a maximum of 100 g, you will need to weigh the ice in two batches of 50 g. Remember to tare the scale with the empty cup before each 50 g measurement.

Figure 6. Ice in cup

3. Ensure that the thermometer and stopwatch are nearby.

4. Pour 100 mL of tap water into the cup, using the graduated cylinder to pour the water in 25 mL at a time.

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Experiment Colligative Properties and Osmotic Pressure

Note: Though some minerals are in the tap water, if you are using ice, the tap water from the ice will get into the water. Since colligative properties are relative to the amount of solute in the solution, you can use tap water and just compare the data from the increase in solute relative to the data from tap water alone.

5. Begin timing for this exercise.

6. Every 30 seconds, record the temperature of the water near the ice.

a. Stir the solution continuously so the temperature is even throughout the cup.

b. When measuring the temperature, keep the thermometer near the ice in the cup, remembering that ice floats in water.

7. Record the temperature at each time point (every 30 seconds) in Data Table 2 of your Lab Report Assistant.

8. After recording is completed, empty the water and ice into a sink.

9. Fully dry the plastic cup and repeat Step 2, weighing 100 g of ice in the cup.

10. Pour 0.5 tsp (2.5 mL) of salt into the plastic cup over the ice and stir. Ensure that when you stir the mixture, you also stir near the bottom of the cup to dissolve all of the salt into the solution.

11. Repeat Steps 4-9. When the water is poured into the cup, stir the water and salt mixture. Record all data in Data Table 2.

12. Rinse the cup well with tap water a few times, pouring out the rinse water each time.

13. Fully dry the plastic cup and repeat step 2, weighing 100 g of ice in the cup.

14. Pour 1.0 tsp (5 mL) of salt into the plastic cup over the ice and stir. Ensure that when you stir the mixture, you also stir near the bottom of the cup to dissolve all of the salt into the solution.

15. Repeat Steps 4-9. When the water is poured into the cup, stir the water and salt mixture. Record all data in Data Table 2.

16. Make a graph of your data for all 3 columns of Data Table 2. Insert the graph in Data Table 3 of your Lab Report Assistant.

● Plot temperature on the y-axis and time on the x-axis

● Input your data from the tap water and saltwater solutions

● The 5 consecutive readings indicate the freezing point for the solution

Questions A. Describe the three freezing points. Is there a relationship between the amount of solute in the

solution and the freezing temperature?

B. What are some practical applications of freezing point depression?

Experiment Colligative Properties and Osmotic Pressure

Exercise 3: Colligative Properties: Boiling Point In this exercise, you will determine the boiling point of tap water, and 2 solutions with tap water and varying amounts of salt added to them.

Procedure

1. Gather the following materials and set up near a source of tap water: 250 mL beaker, burner stand, burner fuel, matches, aluminum cup, pie pan, salt, measuring spoon for a 0.5 teaspoon (2.5 mL), thermometer, stopwatch, and 25 mL graduated cylinder.

2. Using the 25 mL graduated cylinder, measure 100 mL of tap water (in 25 mL increments), and pour the water into the 250 mL beaker.

3. Assemble the burner setup and light the fuel, as shown in Figure 7.

● Place an aluminum pie plate on a solid work surface away from flammable objects. Set the burner stand towards the back of the pie plate.

● Uncap the burner fuel and set cap aside. Place the burner fuel on the pie plate just in front of the stand.

● Use matches or a lighter to ignite the fuel. BE CAREFUL the flame may be nearly invisible.

● Gently slide the fuel under the stand without disturbing the beaker.

● The small, 2 oz. aluminum cup will be placed over the fuel to extinguish the flame. Set the aluminum cup next to the burner setup so you are ready to extinguish the flame at any point.

Figure 7. Burner fuel setup.

4. When the water is at a rolling boil, stir the water with the thermometer and take the temperature near the middle of the 250 mL beaker.

Experiment Colligative Properties and Osmotic Pressure

5. Record the temperature in Data Table 4 of your Lab Report Assistant, in the Temp (°C) of Control column.

6. Use the small, 2 oz. aluminum cup to extinguish the burner fuel flame. See Figure 8.

● Do not touch the metal stand or the beaker; they may be hot.

● Carefully slide the burner fuel canister out from underneath the burner stand. The sides of the burner fuel canister will be warm, but not hot.

● Place the aluminum cup directly over the flame to smother it. The cup should rest on top of the fuel canister, with little or no smoke escaping. Do not disturb the burner stand and beaker; allow everything to cool completely.

Figure 8. Extinguishing burner.

7. Once the beaker has cooled, pour the water from the beaker down the drain.

8. Fully dry the beaker and repeat Step 2.

9. Pour 0.5 tsp (2.5 mL) of salt into the beaker with water in it, and stir the mixture well with the measuring spoon or thermometer until the salt has dissolved into the mixture.

10. Repeat Steps 3-7. Record data in Data Table 4.

11. Rinse the beaker well with tap water a few times, pouring out the rinse water each time.

12. Fully dry the beaker and repeat Step 2. Do not light the burner yet.

Experiment Colligative Properties and Osmotic Pressure

13. Pour 1.0 tsp (5 mL) of salt into the beaker with water in it, and stir the mixture well with the measuring spoon or thermometer until the salt has dissolved into the mixture.

14. Repeat steps 3-7. Record data in Data Table 4.

Cleanup:

15. Wash the beaker with soap and water.

16. Return all items to the kit for future use.

Questions A. Compare the three boiling points. Is there a relationship between the amount of solute in the

solution and the boiling temperature?

B. What are some practical applications of boiling point elevation?