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| Introduction
Special equipment keeps track of the atmospheric pressure and the different gases on the space station, including those produced by the Astronauts themselves. You have learned how scientists measure atmospheric pressure using millimeters of mercury (mmHg). You know that standard temperature and pressure, or STP, is a “perfect” condition in the Earth’s atmosphere, at sea level, in which the pressure is exactly equivalent to 760 mmHg and the temperature is exactly 59( Fahrenheit. Specially designed equipment keeps track of the atmospheric pressure on Space Station Alpha. If it is close to “normal” the Astronauts can do their jobs. Along with the total atmospheric pressure, it is important to keep track of the mix of all the gases. If there is too much oxygen, or too little nitrogen, or too much carbon dioxide, or even the slightest amount of carbon Monoxide in the atmosphere, the Astronauts lives could be in danger. If the total atmospheric pressure in the Space Station is measured in mmHg, can we measure each separate gas in terms of mmHg? If we could, it would help us keep track of potentially dangerous conditions on Space Station Alpha. To learn how to monitor the gaseous content of the atmosphere using mmHg, we will conduct a thought experiment. You will use your knowledge of atmospheric pressure and what you learned about the gases in the atmosphere from your pie graphs. If you want to refresh your memory regarding the molecular composition of gases, take a second to reread The Cube [Ed. Note: Link to The Cube.] The Thought Experiment Equipment – • Graph A, your original Atmospheric Pressure vs. Altitude graph. • A pencil • A straight edge or ruler • Blank paper (white or color) • Scissors • Atmospheric Gases Pie Graph Preparation – 1. On your original, Graph A, Atmospheric Pressure vs. Altitude graph, draw two vertical lines from the x-axis to an altitude of 130,000 ft. Draw one line up from 760 mmHg, or sea level. Draw the second line from the point on the Atmospheric Pressure vs. Altitude curve at which the Atmospheric Pressure is _ STP (380 mmHg). 2. Imagine: Each line represents a hollow tube of air, 1 cm square. (Think: The total pressure of all of the molecules in the tube rising from sea level creates the same pressure as 760 mm of Mercury of Toricelli’s tube.) 3. Imagine and Answer: From the story The Cube how many molecules of air are in the bottom cubic centimeter of the tube drawn from sea level, or 760 mmHg? Answer:_______________________ (2.688 x 1019) 4. Imagine and Guess: How many molecules of air are in the bottom cubic centimeter of the tube drawn at 380 mmHg? Guess:______________________. The quantity of molecules is directly related to the pressure of the atmosphere. With the temperature constant, the pressure depends completely upon the number of gas molecules in the cube. We know that where we find one half the atmospheric pressure of sea level, we will find only one half the molecules in the air. Makes sense, doesn’t it? If there is half as many molecules banging around, there is half the pressure. Let’s check out your guess. 2.688 x 1019 divided by 2. 2/26,880,000,000,000,000,00 = 13,440,000,000,000,000,000 or 1.344x1019 Note: Scientists use the notation 10x power instead of all those “0’s”. It’s called scientific notation. 5. Draw two cylinders, or tubes, that have exactly the same volume as the Cube. On the blank paper, you will draw the first cylinder, or tube, in the shape of an oval and then copy it to make a second cylinder. a. On the blank paper, draw a 35.68 mm line that will represent the horizontal circumference of your cylinder. b. Draw a vertical line of 17.84 mm centered at the center of the horizontal circumference. c. Connect the ends of the two lines to make an oval. d. Turn you oval into a cylinder. To complete the cylinder, or tube, draw a second edge on the oval, one millimeter below the lower half of the oval. Connect this to the ends of the 35.68 mm circumference. You have a cylinder. You are looking at it from a _ view. e. The volume of this one mm high tube, or cylinder, is exactly 1 cm3! Let’s check the math. • Volume of our Cube: V = l x h x w = 1 cm3 = 10mm x 10 mm x 10 mm = 1000 mm2 • Volume of our Cylinder: V = (( r2)h = (3.1418 x 17.842)1mm = 1000.039 mm2. (17.84mm is one half of 35.68mm.) f. Make a duplicate copy of your cylinder so that you have two of them. Draw the horizontal and vertical circumferences and the 1 mm height on the second cylinder. g. On both cylinders, erase the bottom half of the vertical line. 1. On the top of each of your two cylinders, copy the Atmospheric Gases Pie Graph #1 including the chemical symbol for Oxygen and Nitrogen and the percentage of each. In the small “slice” representing the other gases, just include the chemical symbol for Carbon Dioxide and its percentage from the table. 2. Place one cylinder at the bottom of each of the “tubes” on your graph so that it looks like it is “on” the vertical tube. 3. You are now ready for your thought experiments. Thought Experiment Procedure 1. Let’s consider the Cylinder at sea level, first. We know that the total pressure created by all of the gas molecules against the side of our cylinder is exactly 760mmHg, or 100% of the pressure at STP. How much of that pressure is caused by the Nitrogen molecules? By the Oxygen molecules? If 100% of the pressure of all the molecules is 760 mmHg, you can fill in this table: % Composition equals
2. Did you notice the “pp” that was placed in front of the nitrogen’s pressure? The “pp” stands for “partial pressure.” Partial pressure, or pp, is a “signal” to everybody who reads this pressure measurement that the gas’s pressure you are talking about is the pressure of just one of a mix of gases – or a part of a larger pressure created by a mix of gases. 3. Move the cylinder that is on the “tube” starting at _ atmospheric pressure, up so that it is perfectly level with the tip of Mt. Everest. What is the total air pressure in this cylinder at this altitude? What percentage of STP is this? Move it to the height which represents exactly 1/3 the atmospheric pressure of STP, or 253.33 mmHg. Is this close to the top of Mt. Everest? Imagine the molecules of atmosphere in this cylinder at this altitude. 4. Consider: Two cylinders, one at sea level, one close to the tip of Mt. Everest. One contains one third the molecules as the other. What does this tell us about all of the molecules in our Earth’s atmosphere? (Answer: Two thirds of the molecules in the atmosphere around the Earth are below the top of Mt. Everest.) Does this say anything about how important it is to keep this atmosphere clean? Does it say anything about how important it is to keep the atmosphere in Space Station Alpha clean? 5. Bonus Thought: What is the pp (partial pressure) of the Oxygen at the tip of Mt. Everest? Answer: ____________________________ (.20946 x 253.33 mmHg = ) • Recall the Hypoxia Chart: Would climbers at the top of Mt. Everest have to worry about Hypoxia? (Yes) How would they avoid hypoxia? (oxygen masks) Conclusion With your cylinders, air pressure vs. altitude curves, and Toricelli tubes full of air, you can calculate air pressure and partial pressure at any altitude or any atmospheric pressure. On Space Station Alpha the altitude does not change, but the atmospheric pressure and partial pressure of gases can – if we are not careful. Since a change in atmospheric pressure is the same as a change in altitude, we can appreciate what might happen to the Astronauts if the atmospheric pressure in the Space Station were to drop suddenly so that it was similar to their being on the top of Mt. Everest. |
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