Lab 3 Colligative Properties PDF

Preview

Summary

Download Lab 3 Colligative Properties PDF

Description

EXPERIMENT 3 Determining Molecular Weight by Freezing-Point Depression Objectives The student will:   

Learn about colligative properties by observing changes in melting point of a pure material when different concentrations of an unknown solute are added. Determine the molecular weight of an unknown solute from the molality of a solution. Identify an unknown by determining its molecular weight.

Overview The interactions between solute particles and solvent particles influence many of the properties of solutions, such as density, viscosity, surface tension, vapor pressure, boiling point and freezing point, along with osmotic pressure. When a nonvolatile solute is dissolved in a solvent, the solvent’s boiling point is increased, its freezing point is decreased, and its osmotic pressure is increased. Any property that is affected by the amount of solute particle present is called a colligative property. Colligative properties are particularly interesting since the identity of the solute does not affect the observed change in the physical property of the solution. Rather, the relative amount of solute dissolved in the solvent affects the physical property of the solution. The higher the concentration of dissolved solute, the greater will be the observed effect upon the observed vapor pressure, freezing point and boiling point of the solvent. Colligative effects are observed in many phenomena such as the melting of ice on winter roads from “salting”, addition of antifreeze to a car’s radiator to prevent “freeze up”, and the rising of water in trees, due to osmotic pressure. In this lab, we will study the colligative property of freezing point depression. The relationship between the freezing point depression and the corresponding change in temperature is given by the following equations: °

T f =T f −K f ∙ m∨∆ T f =K f ∙m Where, ∆Tf is the difference between the freezing point of the solution and the freezing point of the solvent. The concentration of the solution is determined in molality, m. The molal freezing point constant, Kf, is specific for the solvent being used. Some representative values are shown below. Solvent Water Benzene Camphor Steric Acid

Kf (K/m) 1.86 5.10 5.95 4.50

One use of colligative properties is the determination of the molecular weight of a substance. In this experiment, the freezing point of pure stearic acid will be measured. While steric acid is a solid at room temperature, its relatively low melting point (between 68 and 72 ˚C, depending on purity) allows us to easily convert it to its liquid state and use it as a solvent for an unknown fatty acid. A weighed quantity of an unidentified sample will be added to the pure stearic acid and the freezing point re-determined. The difference between the freezing

points of pure stearic acid and the mixture is the freezing point depression, ∆Tf. Knowing the molal freezing point constant for stearic acid (4.5˚C m-1), the molecular weight of the unidentified sample can be determined by using the following equations. Equation 1

molality=

T f solvent −T f mixture K f solvent

Equation 2

moles of unknown added=(molality of unknown)(kg solvent used ) Equation 3

molecular weight=

total grams of unknown used moles of unknown added

Pre-Lab Questions 1. What is the definition of a colligative property? Describe three colligative properties. Refer to Chapter 16 in your text for help. 2. The formula to calculate molality is:

molality=

moles of solute . What is the molality of an kilograms of solvent

aqueous glucose (C6H12O6, MM=180.16 g/mol) solution that contains 485 g of glucose in 2.00 liters (2.00 kg) of water? 3. The freezing point depression constant of pure water is 1.86 K∙m-1, and the normal freezing point for pure water is 0˚C (273 K). What is the freezing point depression for the glucose solution that you calculated the molality for in #2 above? 4. Read through the procedure below and identify which substance is the solute and which substance is the solvent in this experiment. * Make sure your data tables are prepared BEFORE coming to lab. Sample data tables can be found on page 4.*

Procedure 1. Prepare a warm water bath: Fill a 600 mL beaker 3/4ths full with hot tap water. Add a magnetic stir bar. Place on the hot plate with the stir bar on low (water takes a while to heat). You do NOT want the water to boil. The goal is to get the water to a constant temperature of 85 °C. This may require some adjustment. Position a ring stand with a utility clamp over the water bath so that a test tube can be clamped in the water bath. 2. Prepare a room temperature water bath: Add about 300 mL of tap water with a temperature of 20-25 °C (use a standard thermometer to check) to a 400 mL beaker. Place the beaker on the base of a second ring stand, so that you can clamp the test tube to it for the cooling step. 3. Obtain a 15 x 125 mm (medium sized) test tube. Accurately weigh (to the nearest 0.01 g) approximately 9 g of stearic acid into the 15 x 125 mm test tube. This is best accomplished by first weighing an empty 250 mL beaker containing the clean, dry sample test tube and recording the mass. Subsequently transfer about 9 g of

stearic acid to the sample tube and weigh again to obtain the mass of the stearic acid. Record the exact masses in your data table. 4. Secure the tube containing the steric acid to in the warm water bath using the utility clamp. After the fatty acid sample has completely melted, place the temperature probe in the tube. Be careful that the cord of the probe does not touch the hot plate. Feel free to use a thermometer clamp to secure the cord, if necessary. 5. Connect the Temperature Probe to LabQuest and choose New from the File menu. 6. On the Meter screen, tap Rate. Change the data-collection rate to 1 sample/second and the data-collection duration to 600 seconds. 7. Once the sample has reached 85 ˚C, quickly, remove the test tube by removing the clamp, transfer the test tube to the room temperature water bath and IMMEDIATELY start data collection, clamping the tube to the ring stand as shown in figure 1 below. Stir the mixture continuously with a slight up and down motion of the temperature probe for the entire 10 minute data collection period.

Figure 1: Cooling water bath with probe during data collection period 8. When the data collection is complete, return the tube and probe to the hot water bath to re-melt the steric acid while you analyze your data from this first run.  The freezing temperature can be determined by finding the mean temperature in the portion of the graph with nearly constant temperature.  Select the data point at the beginning of the flat portion of the graph and drag across the flat portion to select the region.  Choose Statistics from the Analyze menu.  Record the mean (average) temperature as the freezing temperature of pure stearic acid.  Store the data from the first run by tapping the File Cabinet icon. ( 9. Accurately weigh (to the nearest 0.01 g) approximately 1 g of an unknown fatty acid onto weigh paper. Record the letter of your unknown and its exact mass in your data table. 10. Carefully transfer ALL of your unknown to your melted steric acid and stir with the temperature probe to fully combine/melt. Keep the test tube in the warm water bath until the solution reaches 85˚C.

11. Before continuing, use the standard thermometer to ensure that the temperature of your cool water bath is still between 20 and 25 °C. If not, replace the water.

12. Once the sample has reached 85 ˚C, quickly, remove the test tube, start data collection, and clamp the test tube in the previously prepared room temperature water bath, as before clamping the tube to the ring stand as shown in figure 1 below. Stir the mixture continuously with a slight up and down motion of the temperature probe for the entire 10 minute data collection period, as before.

13. When the data collection is complete, return the tube and probe to the hot water bath to re-melt the solution while you analyze your data from this second run. Examine the data pairs on the displayed graph. The freezing point of the solution can be determined by finding the temperature at which the mixture started to freeze. Unlike pure steric acid, the mixture results in a gradual linear decrease in temperature during freezing. As you move the examine line, the temperature and time values are displayed to the right of the graph. Locate the initial freezing temperature of the solution, as shown below. Record the freezing temperature in your data table. Freezing Point

Time

14. Tap Run 2 and select All Runs to view both trials on the same graph. 15. Repeat steps 9-14 two more times with 1 g additions of your unknown (a total of 3 grams will be added) 16. Print the resulting graph directly from the Lab Quest. 17. Return the test tube to the warm water bath to melt the steric acid solution one more time. Remove the temperature probe and immediately wipe with a paper towel. A warm isopropanol bath has been set up in the hood if the probe needs cleaned further. 18. Pour the melted steric acid solution into the waste container in the hood (NOT DOWN THE DRAIN!). Place the test tube in the collection container in the hood.

Data: Record your data in a data table IN YOUR NOTEBOOK which can be a LARGER version of the table shown here Table 1: Data

Substance Steric Acid Freezing temp of pure steric acid 1st addition of unknown Freezing temp after 1st addition 2nd addition of unknown Freezing temp after 2nd addition 3rd addition of unknown Freezing temp after 3rd addition

Mass

Analysis: ***For each prompt below, show a sample calculation in your lab notebook and record your answers for each trial in a table like the one shown (in your notebook). 1. (3 points) Calculate ΔTf for each of the three solutions. This value is found by taking the new freezing point and subtracting the freezing point of the pure solvent.

2. (3 points) Using the values you just calculated for ΔTf, calculate the experimentally determined molality of the three solutions that contained the unknown. Use the equation ΔTf =Kf ∙ m. The freezing point depression constant for stearic acid is Kf == 4.5˚C m-1. Note: Do not calculate the theoretical molality (molality=mol/kg) 3. (3 points) Using the experimentally determined molality (m) determined in #2, calculate the moles of unknown solute added using the equation below. m=

moles unknown added ∨moles unknown added= ( m) ∙ (kg solvent added ) kg solvent added

4. (3 points)Using the value for moles of solute calculated in #5, determine the molecular weight (MW) of the solute using the equation below. Show a sample calculation below and record all your answers in the data table on the following page.

MW =

grams of unknown used moles of unknownadded

5. (1.5 points) Determine the average molecular weight of your unknown solute. Table 2: Calculated values: ΔTf values (#1)

Experimentally dermined molality (m) (#2)

Moles of Unknown Solute Added (#3)

Molecular Weight of Unknown Solute (#4)

Average Molecular Weight(#5)

First Addition of Unknown Second Addition of Unknown Third Addition of Unknown

6. (1.5 points) Your unknown substance is one of the three fatty acids listed in the table below. Determine the molecular weight, to two decimal places, for each of the following compounds. You can copy a larger version of the table below into your lab notebook. Table 3: Identity of Unknowns Compound Lauric acid Myristic acid Palmitic acid

Molecular Formula C12H24O2 C14H28O2 C16H32O2

Molecular Weight (g/mol)

7. (3 points) Compare your experimentally determined average molecular weight (#7) to the molecular weight of the substances in the table above. What is the likely identity of your unknown? 8. (2 points) Determine the percent error of the experimentally determined average molecular weight compared to the actual molecular weight. The formula used to calculate experimental error is below.

%Error=

|Experimental Value− Accepted Value | Accepted Value

∙ 100

Note: The “Accepted Value” is the molecular weight you calculated in #7

Conclusion: Write your conclusion in your lab notebook. Discuss possible error sources in your experiment.

*Adapted From: McCarthy, S.; Gordon-Wylie, S. W. J.Chem. Educ. 2005, 82, 116-119....