AP/IB Chemistry Lab Name:
Molar Mass (Mr) of a Volatile Liquid (DCP, CE)
INTRODUCTION
In this experiment, an amount of liquid more than sufficient to fill the flask when vaporized is placed in a flask of measured volume and mass. The flask is then heated in a boiling water bath so that the liquid vaporizes completely. The vapor drives air out of the flask and fills the flask at barometric pressure and the temperature of the water bath, both of which can be measured. The pressure is low enough and the temperature is high enough for the gas to behave ideally. The flask is weighed after cooling to condense the vapor. You should then be able to determine the molar mass (molecular weight). Repeat your experiment to assess reproducibility.
MATERIALS
125 mL flask 15 x 125 mm test tube 1-hole rubber stoppers volatile organic liquid (methanol, ethanol) 600 mL beaker (1000 mL works great) tongs thermometers barometer analytical balance
EXPERIMENT DESIGN1: 1. Obtain and wear goggles. Conduct this experiment in a fume hood or well-ventilated area. 2. Prepare a hot-water bath by warming about 300 mL of tap water in a 400 mL beaker. Keep the beaker on the hot plate once the water is warm. 3. Trim a piece of aluminum foil so that it just covers the top of a small, 13 x 100 mm, test tube. Use a needle to make a small hole in the foil. Use a sensitive balance to measure the mass of the test tube and foil. 4. Use a second 400 mL beaker to prepare an ice-water bath or use a large ice water bath for the class. 5. Obtain a liquid sample of an unknown volatile compound. Pour about 0.5 mL of the liquid into the test tube and quickly cover the test tube with the aluminum foi8l. Place the test tube in the hot-water bath. Make sure that the foil is above the water level. 6. Immerse the thermometer or temperature probe in the hot water bath. Do not allow the tip of the probe to touch the beaker. This will give you a more accurate reading of the water bath temperature.
1 Adapted from Advanced Chemistry with Vernier. For educational purposes only. 7. Record temperature data by hand or using data logging equipment with temperature probe. 8. Heat the beaker of water to boiling and maintain the boiling as your sample of liquid vaporzes. Note that some of your sample will escape the test tube through the needle hole in the foil. This process also serves to flush the air out of the test tube. 9. Keep the test tube in the boiling water bath for at least three minutes after all the liquid in the test tube has vaporized. Record final temperature / stop collecting data using data logging equipment. Examine your temperature graph and record the temperature of the boiling-water bath, which will be used later in calculations. 10. Use a test-tube holder to quickly transfer the test tube to the ice water bath. Cool the test tube for about one minute, then remove it and dry it completely. Measure the mass of the test tube and the aluminum foil top. 11. Record the room barometric pressure. 12. Rinse out the test tube and fill it to the top with water. Cover the test tube with the aluminum foil. Measure and record the mass of the test tube, water, and foil. 13. Clean up!
PRE-LAB ASSIGNMENT (2 parts):
Part 1 (on a new right hand page): Write an appropriate introduction and problem statement, sketch a labeled diagram of the apparatus you expect to assmeble, and prepare your notebook for data collection. This work should also be done in the class notebook (on a new right hand page if necessary).
Part 2 (on left hand page opposite pg 1): After reading through the lab handout and procedures, answer the following questions on the left hand page in your class notebook. Answer in complete sentences and show all work as appropriate.
1. Draw particle diagrams representing the volatile liquid in the flask before heating and near the end of being heated. 2. What is an ideal gas? Can the vapor of the liquid in this experiment be assumed to behave as an ideal gas? 3. Describe two methods for determining the volume that the vapor occupies in this experiment. Which method is likely to give more precise results? Why?
REPORT / ASSESSMENT
Report all collected data in appropriate DCP format. You will be formally assessed on your data collection and processing (DCP) and conclusions and evaluations (CE) for this lab report. Your lab report should include a problem statement/brief introduction in place of the Design section. Also include a paragraph summary of the procedures you actually did do in the lab, addressing changes you made to the given procedures (though this will not be assessed for IB scoring).
4. If 1.91 g of the vapor of a volatile liquid is able to fill a 408-mL flask at 100.4 oC and 765 mm Hg, what is the molar mass of the liquid? AP/IB Chemistry Pre- / Post- Lab* Name:
Molecular Weight of a Volatile Liquid
Pre-Lab Assignment (2 parts):
Part 1: After reading through the lab handout and procedures, answer the following questions in your lab notebook. Answer in complete sentences and show all work as appropriate.
1. Draw particle diagrams representing the volatile liquid in the flask before heating and near the end of being heated. 2. The method used in this experiment is sometimes called the vapor density method. Beginning with the ideal gas equation, show how the density of vapor may be determined by this method. 3. If 1.91 g of the vapor of a volatile liquid is able to fill a 408-mL flask at 100.4 oC and 765 mm Hg, what is the molar mass of the liquid? What is the density of the vapor under these conditions? 4. What is an ideal gas? Can the vapor of the liquid in this experiment be assumed to behave as an ideal gas? 5. Describe two methods for determining the volume that the vapor occupies in this experiment. Which method is likely to give more precise results? Why?
Part 2: Complete an appropriate problem statement introduction and prepare for your data collection for this lab. This work should also be done in the lab notebook. Post-Lab Questions:
Complete the following questions on your own piece of paper (not in lab notebook, may be typed). This work is separate from the lab report and will be submitted as its own assignment. Answer in complete sentences, showing work or mathematical reasoning where appropriate.
1. It was important that the flask used in this experiment be completely dry before the unknown liquid was added, so that any water present would not vaporize when the flask was heated. A typical single drop of liquid water has a volume of approximately 0.05 mL Assuming the density of liquid water is 1.0 g/mL, how many moles of water are in one drop of liquid? What volume would this amount of water occupy when vaporized at 100 oC and 1 atm? What error would this introduce into the determination of the molar mass of the unknown? 2. The pressure of the vapor of the unknown liquid in this experiment was taken to be equal to the atmospheric pressure in the laboratory. Why?
*Questions adapted from “Experimental Chemistry” by Hall (accompanies 7th ed. Zumdahl)