Molecular Mass Determination by Boiling Point Elevation

Transcription

1 Exercise 15 Page 1 Illinois Central College CHEMISTRY 130 Name: Molecular Mass Determination by Boiling Point Elevation Objectives. In this experiment we will use the fact that the presence of a nonvolatile solute in a solution causes its boiling point to rise proportionate with its molality. By measuring the amount of boiling point elevation, we can determine the molecular mass of the solute. Background The colligative properties of solutions are those properties that depend on solute concentration. These properties include: boiling point elevation freezing point depression osmotic pressure vapor pressure reduction Each of these properties is directly proportional to solute concentration. In today's lab we will look at the effect on boiling point due to the presence of a nonvolatile, nonelectrolyte solute. The relationship between solute concentration and boiling point elevation is defined by the following expression. T = i k b m (1) where "i" is defined as the van't Hoff Factor (the number of particles a solute dissociates into in solution...which for a nonelectrolyte, is 1), "k b " is the boiling point elevation constant characteristic of the solvent being used (for water, k b is o C/m) and "m" is the molality of the solute concentration (that is, moles of solute per kilogram of solvent). Since molality is defined as (1) above to obtain: m = moles of solute kilograms of solvent, we can substitute this expression into the equation T = i k b ( moles of solute ) kilograms of solvent (2) We can further expand this equation by using the fact that moles of solute is defined as: moles of solute = grams of solute molecular mass of solute

2 Exercise 15 Page 2 By substituting this expression into equation (2) we obtain the following equation: T = i k b ( ( grams ofsolute molecular mass ofsolute ) kilograms of solvent ) (3) If we solve equation (3) for Molecular Mass we obtain: Molecular Mass= i (k b) (grams of solute) (kilograms of solvent) ( T) A careful examination of this equation reveals that we simply dissolve a measured quantity of solute (in grams) into a measured quantity of solvent (in kilograms) and measure the change in boiling point in order to determine the Molecular Mass of the unknown solute. Procedure 1. Connect the Go-LinK to the USB connestor of your computer and connect the Stainless Steel Temperature Probe to the Go-LinK. 2. Start the Logger Pro program on your computer. Open the file Boiling Point Elevation from the Chemistry with Computers folder. You should now see the screen shown here. This file allows you to collect temperature readings at half-second intervals and display them graphically. Thermistor Calibration CHECK WITH YOUR INSTRUCTOR TO SEE IF CALIBRATION IS NECESSARY In most cases, the Stainless Steel Temperature Probe will never need to be calibrated. However, if the need arises the sensor can be custom calibrated. You will need a good thermometer and three containers of water at three different temperatures. 1. Open Logger Pro and from the Menu Bar choose Experiment/Calibrate and then choose the Temperature Probe.Choose Calibrate Now. For each of the three water baths, place the temperature probe in the bath with the thermometer. 2. Allow both readings to stabilize, click, and enter the temperature reading measured by the thermometer. 3. Click Done after the third water bath reading.

3 Exercise 15 Page 3 Note: If you want the calibration to be saved with the file for later use after you calibrate, you need to do two things: Go to the calibration dialogue box and choose Calibration Storage from the pull down menu. Then set the Calibration Retrieval Preference to Experiment file. Save the Experiment file. Running the Experiment To obtain a baseline boiling point, we will measure the boiling point of pure water first, then introduce a measured amount of ethylene glycol (a nonelectrolyte) and measure the new 'elevated' boiling point. In order to illustrate the effect of the van't Hoff Factor, we will also measure the elevated boiling point of a NaCl solution of identical molality (which ionizes into twice as many solute particles as the nonelectrolyte, ethylene glycol). The Baseline Boiling Point Temp Probe 1. Obtain a 250 ml round-bottom florence flask. Using a graduated cylinder, measure 100 ml of distilled water (Density=1.00 g/ml) into the flask. Place 3 or 4 boiling chips in the flask to ensure smooth boiling. Fit the flask with the Stainless Steel Temperature Probe, adjusting the height of the probe such that the tip is approximately half-way to the bottom. (See Figure 1.) 2. Turn the hotplate temperature to the maimum and watch the temperature displayed on the screen. When the temperature approaches 70 o C, click the Collect button on the Logger Pro Tool Bar. Fig The program will wait for a triggering temperature of 80 o C before it begins plotting. Once the temperature reaches 80 o C, allow the program to collect data until a boiling "plateau" has been well established as in the screen shown here. At this point you can click the Stop button on the Tool Bar.

4 Exercise 15 Page 4 Elevated Boiling Point 1. When the first plot has been completed, empty the flask and obtain a fresh 100 ml of distilled water rather then waiting for it to cool. 2. To the water in the flask, add 22.5 ml ethylene glycol (Density=1.10 g/ml) and 3 or 4 boiling chips. Mix well and begin heating. As before, when the temperature approaches 70 o C, click the Collect button on the Tool Bar. You will be prompted to choose how to handle the previous data collection. Choose Store Latest Run. 3. As before, the program will begin plotting points when the temperature reaches 80 o C. When the plot has established a good boiling "plateau" click the Stop button on the Logger Pro Tool Bar. You should now see both plots on the screen as shown here. Abnormal Boiling Point Elevation 1. When the second plot is complete, again obtain a fresh 100 ml sample of water and introduce 23.5 grams NaCl and 3 or 4 boiling chips. This is sufficient NaCl to produce the same molality as was used in the ethylene glycol determination. The difference here, however, is that NaCl is an electrolyte with a van't Hoff Factor of approximately '2'. We should see a significantly larger (almost twice as great) boiling point elevation in this case. 2. Mix the NaCl until it is completely dissolved, and begin heating. As before, when the temperature approaches 70 o C, click the Collect button on the Tool Bar. You will be prompted to choose how to handle the previous data collection. Choose Store Latest Run. 3. As before, the program will begin plotting points when the temperature reaches 80 o C. When the plot has established a good boiling "plateau" click the Stop button on the Logger Pro Tool Bar. You should now see all three plots on the screen as shown here.

5 Exercise 15 Page 5 Data Analysis 1. Scroll through the data for your three experiments and estimate the boiling temperature of the three solutions to the nearest tenth of a degree. Record the boiling point of the pure water and the boiling point of the water/ethylene glycol mixture on your Report Sheet. 2. Determine the change in boiling point ( T) between the pure water and the water/ethylene glycol mixture. Use this T to determine the Molecular Mass of ethylene glycol. 3. Select File/PrintGraph and print 2 copies of this plot to be attached to your lab reports. 4. No calculation is required with the water/nacl plot. It is displayed merely to demonstrate the effect of the van't Hoff Factor in boiling point elevations. After you have printed your graphs, exit the program.

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7 Exercise 15 Page 7 Illinois Central College CHEMISTRY 130 Name: REPORT SHEET Molecular Mass Determination by Boiling Point Elevation Ethylene Glycol Determination Mass of water used (grams) Boiling point of pure water ( o C) Volume of ethylene glycol used (ml) Mass of ethylene glycol used (grams) Boiling point of ethylene glycol/water mixture ( o C) ( T), Boiling point elevation ( o C) Molecular mass of ethylene glycol (g/mol) Calculations: Abnormal Boiling Point Elevation Mass of water used (grams) Mass of salt used (grams) Molality of salt solution Boiling point of salt solution ( o C) ( T), Boiling point elevation ( o C) Which of the solutions exhibited the greater boiling point elevation? Explain why.

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9 Exercise 15 Page 9 Illinois Central College CHEMISTRY 130 Name: SHOW YOUR WORK PRELAB: Exp.15 Boiling Point Elevation 1. A solution contains 5.60 grams of urea (a nonelectrolyte), CON 2 H 4, in 50.0 grams of water. Calculate the molality of the solution. 2. Calculate the boiling point of the solution in Problem #1. (k b for water is o C/m) 3. Calculate the freezing point of the solution in Problem #1. (k f for water is 1.86 o C/m) 4. A solution containing grams of an unknown nonelectrolyte dissolved in grams of water boils at o C. What is the molecular mass of the unknown substance?

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