Lab # 9 – Does a Gas Have Mass?
Gases are
usually colorless and odorless. In most cases it is difficult to perceive that
equal volumes of gases, at the same temperature and pressure, contain the same
number of molecules. Do these invisible molecules really have appreciable masses
that can be measured? What happens if you squeeze additional volumes of gases
into a container? Does the mass increase? What happens to the pressure of the
gases? Is the increase in the mass related to the increase in the pressure?
OBJECTIVES
· Record volume of gas added vs. mass data as gas is added to a rigid container.
· Record volume of gas added vs. pressure as gas is added to a rigid container.
· Graph pressure vs. mass.
MATERIALS
|
computer |
250 mL
Erlenmeyer flask |
|
Vernier
computer interface |
safety
goggles |
|
Logger
Pro |
20 cc syringe |
|
Vernier Gas Pressure Sensor with plastic tubing |
2 holed stopper 2
2-way valves |
PART 1
PROCEDURE
1. Obtain and wear goggles.
2. Insert a rubber stopper with 2 2-way valves into the top of a 250 mL Erlenmeyer flask.
3. Tare Erlenmeyer flask/stopper/valves to zero.
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4. Open Logger Pro 3.3.
5. Double click in the <x> box (column definition) and label it “volume added”, put “mL” for units, then click <done>.
6. Double click on the <y> box (column definition) and label it “mass”, put “g” for units, then click <done>.
7. Enter “0” for <volume>, “0” for <mass>.
8. With the assistance of one of your partners, pull back syringe to 20 mL, connect to the valve, person (1) open the valve, person (2) should depress the syringe, and then person (1) closes the valve. Then weigh the flask.
9. Record data in computer (volume added = x, mass = y).
10. Repeat four times, each time add 20 mL. Record data as you each additional 20.0 mL of air.
DO NOT EXCEED
80 mL in the flask!
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11. Choose “Analyze”, “auto scale” and a graph will appear.
12. Open the valve and release the pressure to prepare for the next part.
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PART 2
PROCEDURE
1. Wear goggles.
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2. Prepare the Gas Pressure Sensor for data collection.
a. Connect the Gas Pressure Sensor to Channel 1 of the interface. Attach the tubing to the connector on the Gas Pressure Sensor and to the valve in the rubber stopper (put rubber stopper in the flask, push hard and twist). Make sure the valve to the pressure sensor is open and the other valve is closed. Two more columns will appear on the computer screen.
3. Change the pressure units from Pa (pascals) to atm (atmospheres)
A. Go to
a. Experiment
i. Set Up Sensors
1. Show All Interfaces
B. Double click on CH 1 (Pressure Sensor)
a. Select “atm”
b. Close
C. Double click column heading that says <Time>; change it to <Volume Added> and change the units to mL.
D. Go to the Clock icon at the top of the screen (next to the green <Collect> button)
a. In the Mode pull down menu select <Events with Entry>. You will need to change the column name to VolP (or something different from Volume)
4. On the computer, hit <collect>, then <keep> enter “0” for the <volume added>. (The computer automatically enters the pressure.)
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5. Pull back syringe to 20 mL, connect to closed valve, open valve and depress the 20 mL of air into the flask. Press <keep>, enter “20.0” for the volume. Repeat procedure: pressing <keep>, enter volume, and repeat again, two more times for a total collection of 80 mL.
6. Now hit <stop>
7. Choose <Analyze>, <auto scale> and a new graph will appear.
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8. Now insert a third graph by going to <insert> <graph>. A graph will appear in the middle of the screen. On the x-axis select <mass>. On the y-axis select <more>; deselect <mass> under the <Data Set> option; open the <Latest> option and select <pressure>. Under <scaling> select <Autoscale>.
9. Go to <Page>, select <auto arrange>. All 3 graphs will appear on the screen.
10. To print, go to <page setup>, select <landscape> and hit <print>.
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Questions:
1. Based on the data you gathered, does air have mass? Use your data to support your answer.
-
What is the relationship between the mass and the pressure? Explain, using the kinetic molecular theory, the reason for this relationship.
-
What experimental factor was kept constant in this exercise?
-
Predict what might happen if you had tried to add 200 mL of air to the flask. Explain why your prediction would occur.
-
Describe an experience you have had that illustrates one of the three relationships examined in this lab (mass vs volume, pressure vs volume, mass vs pressure).
Problems - Moles/Pressure & Mols/Volume
- If 1.25 mol of oxygen occupies 20.5 L, what will be the volume of 2.75 mol of oxygen? (Assuming temperature and pressure are constant.)
- If 0.750 mol of nitrogen occupies 1375 mL, how many mols of nitrogen would occupy 995 mL? (Assuming temperature and pressure are constant.)
- If the pressure of 0.350 mols of carbon dioxide is 685 torr, what will the pressure of 0.550 mols of carbon dioxide be? (Assuming temperature and volume are constant.)
- The pressure of 2.98 mol of He is 2.75 atm. If the pressure decreases to 1.35 atm, how many mols of He do you have? (Assuming temperature and volume are constant.)