CHEMISTRY 130 General Chemistry I. The Molar Mass of Air

Transcription

1 CHEMISTRY 130 General Chemistry I The Molar Mass of Air Molar mass is simply the mass of one mole of a substance. As suggested by the picture [1], what about the mass of one mole of the air we breathe? We can measure it in the lab using buoyancy and the Ideal Gas Law. DEPARTMENT OF CHEMISTRY UNIVERSITY OF KANSAS

2 The Molar Mass of Air Introduction The air we breathe, the carbon dioxide pumped into our soda cans, and the hydrogen balloons that science instructors fondly explode during class are each examples of gasses. The physical behavior of gases can be described by scientific laws that are grounded in hundreds of years of experimentation. This week, we will perform hands-on scientific inquiry an experiment that will help us understand something of the quantitative physical behavior of gases. The experiment serves as a practical review of the most comprehensive gas law you'll study: the Ideal Gas Law. Like many scientific laws, it can be succinctly stated in equation form: PV=nRT. The definitions of the symbols are: P = gas pressure V = volume the gas occupies n = number of moles of gas R = the "gas constant," often expressed as L atm/k mol T = temperature of the gas, in Kelvin This equation provides a very useful, general description of the physical behavior of ideal gases. Although real gases do not behave in an absolutely ideal manner, this equation will provide a good basis for the investigation you are performing in the laboratory this week. When any four of the five variables in this equation are known, the equation may be rearranged and easily solved to determine the unknown value. When describing the physical behavior of a gas, it is important to remember that gases are fluids. We are used to thinking of liquids as fluids, but gases are fluids, too. In fact, a fluid may refer to any substance that flows and can change shape easily to "fill" its container. Under so-called "STP" or standard temperature and pressure conditions, corresponding to 0 C (273 K) and 1 atmosphere (1 atm) of pressure, gases exist as discrete isolated particles in large volumes of empty space. A given volume of gas contains a relatively small number of molecules in comparison to the same volume of liquid or solid material. You might say that a gas is less concentrated. By decreasing the volume of the vessel containing the gas, the molecules can be forced closer together (this happens, for example, when a tire pump is used to compress air to fill a tire). As you might expect, gases are called compressible fluids, whereas liquids are described as incompressible fluids. Even if you didn't call it "Archimedes Principle," perhaps you recall that fluids exert a buoyant force on objects wholly or partially immersed in them. As such, many objects float on water. But Archimedes Principle applies to any fluid, including air and other gases. Air exerts a buoyant force on objects submerged in it. Balloons that are filled with a gas that is less dense than air will float if the weight of the balloon and the gas inside is less than the weight of the air it displaces. In this experiment, we will use buoyancy and the Ideal Gas Law to measure the molar mass of the air we breathe. 2

3 Pre-lab Safety: Goggles must be worn at all times. Gas cylinders (Figure 1) contain gas that is under high pressure and they should be handled with great care. This type of container is very sturdy and well adapted for the purpose it serves. It MUST remain firmly clamped and strapped into the bracket specifically designed to secure it. NEVER attempt to unfasten or move the cylinder, and do not attempt to adjust the pressure gauge in any way. If the cylinder were to become loose, topple over, and the valve broken, it would function as a rocket! THERE ARE INSTANCES IN WHICH Figure 2 [3] : A pressure regulator and pressure gauge that attach to the gas cylinder to control the flow of gases GAS CYLINDERS HAVE BEEN PROPELLED THROUGH CINDERBLOCK WALLS AND ANYTHING ELSE IN THEIR PATH. Your TA will fill your Mylar balloons using the balloon valve on the gas cylinder. Pre-lab Assignment: Please write out the following in your lab notebook. This assignment must be completed before the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and accepted by your TA. 1) List the chemical you will use for this week's experiment. List specific safety precaution(s) that must be followed. In order to find specific safety information, please obtain a Materials Safety Data Sheet (MSDS) on the chemical of interest. MSDSs can be found through an internet search (e.g., Google) or from the following website: Read the MSDS and find specific safety concerns. 2) Consider the following pairs of substances. Think about the physical behavior of these substances if each pair were placed together in a separate container. Explain what would occur for each set and why. In each case, indicate which substance is denser. a. Salad oil and vinegar b. Gasoline and water c. Oil and water d. Water and ice cubes e. Water and liquid mercury f. A hydrogen-filled balloon and the atmosphere Valve; controls air flow, including shutting the gas off completely. Tank; contains lots of gas in a small volume at high pressure. Outlet; site of gas ejection; can and should be outfitted with a pressure regulator (Figure 2). Figure 1 [2] : A labeled gas tank or cylinder, unsecured with no pressure gauge. 3) Compute the number of moles of helium (He) gas contained in a 22.4-L-size container at 1.00 atm pressure and a temperature of 273 K. Use L atm/mol K for the gas constant. 4) The week of Feb. 29, 2016, PBS aired a documentary on sending humans halfway to space in the late 1950 s using balloons. They filled different balloons with different gases some with H 2 and some with He. Which balloon should go higher? Why? 3

4 Procedure Part 1 The Apparent Mass of a Helium Sample As a group, discuss your answers to Prelab question 1. Did all the teams come to the same conclusion about the pairs of substances? What physical properties are responsible for this behavior? 1. Fill a Mylar balloon with air using a squeeze bulb. When it is nearly full, twist the neck of balloon closed and fasten it using the plastic tie. Using the top-loading balance, obtain the mass of the air-filled balloon. 2. Ask your TA to carefully fill the Mylar balloon with helium. The TA will take care to avoid overfilling the balloon; i.e., the balloon should be almost completely expanded but not stretched tight. When it is nearly full, twist the neck of balloon closed and fasten it using the plastic tie. Obtain the apparent mass of the helium-filled balloon using the top-loading balance. 3. Find the difference in apparent mass, m, between the air-filled balloon and the heliumfilled balloon. 4. Repeat steps 2 and 3 two more times. Find the average m and record your team's value on the board. Discuss the data that has been collected. Is there anything unusual about the change in mass? Is it consistent with your expectations? The physical meaning of m can be represented as m = [n molar mass of air] [n atomic mass of He] Thus, to calculate the molar mass of air, we need values for all of the other variables in the equation. The atomic mass of helium is known. Where might you find it? The experimental data obtained above provides m. However, we are not quite ready to use the equation. We must first apply the Ideal Gas Law as described in Part 2 to obtain n, the number of moles of gas contained in the balloon. 4

5 Part 2 The Ideal Gas Law: Measuring the P, V, and T of the Helium Sample Connect the Vernier Logger Pro system to a power supply and to the computer. Pressure, P: A physical property that you have not yet measured in general chemistry lab is gas pressure the P of PV=nRT. Our Vernier Pressure Sensor will be used to do something that a typical tire pressure gage cannot: measure the current atmospheric pressure. a. Plug the cord from the Pressure Sensor (Figure 3) box into one of the four channels (CH1, CH2, etc.) on the LabPro box. b. Go to the Experiment pull-down menu and select "Set Up Sensors" followed by "Show All Interfaces." ON/OFF valve connecting gas sample to pressure sensor connection to gas sample (balloon) Connection to LoggerPro CH1 or CH2 Figure 3: Pressure Sensor compatible with the Logger Pro system c. A dialog box will open which shows the four channels along with a series of boxes that show which probe is plugged into each port. Right click on the box corresponding to the Channel you're using for the Pressure Sensor. d. Select "Choose Sensor" and follow through the menus until you find "Pressure Sensor." A tiny picture of the Pressure Sensor should appear in the box corresponding to the selected Channel. Temperature, T: Connection to LoggerPro CH1 or CH2 Temperature Probe; placed within the material being measured for temperature. Figure 4: Temperature Probe compatible with the Logger Pro system The temperature of a substance, including the air in the balloon, can be measured using the Temperature Probe (Figure 4) in your lab drawer. The Temperature Probe is connected and initiated similarly to the Pressure Sensor; follow the directions above to connect it to the Logger Pro system and to the computer. 5

6 Volume, V: We do not have any Vernier equipment for direct determination of the volume of an object, but this can be measured using the water displacement technique. (Recall the measuring volume to determine density during your first lab session.) a. Place the overflow tank in the large sink and fill the tank until water just begins to spill out the overflow spout. Wait until water ceases to flow from the spout. b. Place a large, empty container below the spout. c. Place the balloon whose volume you wish to measure into the tank. Gently push it down until it is just submerged. Try not to measure the volume of your hand along with the object! (Be sure that no gas can escape from the neck of the balloon as you are taking the measurement.) Hold the balloon below the surface until water stops overflowing. d. Remove the balloon from the tank. e. Measure the volume of the overflow water, which will be the same as the volume of the submerged object. Part 3 Putting It Together: Calculating the Molar Mass of Air Using your experimental data from Parts 1 and 2, work with your group members and TA to calculate the molar mass of air from the Ideal Gas Law and m equation. Use the accepted value for the average molar mass of dry air, provided by your TA, to calculate the % error in your experimentally determined value for the molar mass of air. This calculation is performed as shown below: accepted value experimental value accepted value 100% = % error What factors are likely responsible for the error in your experimental value? Lab Clean-up: Please make sure your lab area is clean and dry before leaving the lab, with all materials returned to their original positions. Pressure Sensors should remain on the benchtop, with the cords neatly and loosely arranged. 6

7 Post-Lab Questions After completing the lab, answer the following questions in your laboratory notebook. 1) The accepted value for molar mass of air provided by your TA can be estimated by performing a simple calculation. (Recall composition problems from earlier in the semester.) The mole fractions for the 3 most prevalent components of dry air are: Using these data and the molar masses of each component, calculate the molar mass of dry air (no water present). 2) Under common conditions, such as those in the lab, air contains small amounts of water. The nitrogen and oxygen decrease by small amounts say, half a percent each and are replaced by water. a. How would this change your accepted value calculated in problem 1 (increase, decrease, stay the same)? Explain. b. How would this change your % error value (increase, decrease, stay the same)? Explain. (NOTE: this is an exaggeration. It would be very humid under such conditions.) 3) How would your % error change if your hands are fully submerged beneath the water when measuring the volume of the balloon (increase, decrease, stay the same)? Explain. Report Introduction A report approximately 4 pages in length is due at the beginning of next week s lab period. It is a partial, rather than Experimental Procedures full, report and should be prepared and submitted Results individually, rather than in groups. It will consist of the Discussion Introduction, Results, and Discussion sections of a full report. You have practiced writing these sections of a lab report Conclusion already. This is your opportunity to improve based on the feedback you received from your TA. Because you have practiced writing these sections of a laboratory report, please consult previous laboratory reports to see the expectations for each section. For the Results section, please organize your data in one or more tables. This can be done in spreadsheet software, such as Microsoft Excel. Post-lab question 1 has an example of such a table. The Discussion section should include information about how you interpret or contextualize your Results. Writing out or expounding upon the post-lab questions may be a good starting point. Reference(s) [1] accessed Mar. 3, [2] -compressed, accessed Mar. 1, [3] accessed Mar. 1,

8 Glossary Buoyancy the ability or tendency of one fluid to float in or rise above another fluid, based on density; the less dense material rises above or floats on top of the denser material Sensor (Merriam-Webster) a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (as for measurement or operating a control) Ideal gas A hypothetical, idealized gas, in which none of the gas particles exert electrostatic forces on one another, all collisions are perfectly elastic, and the gas particles have no volume (are point particles.); an ideal gas can be described using the Ideal Gas Equation: PV=nRT Mylar a type of plastic sheet or foil made by DuPont and consisting of polyethylene terephthalate (PET) molecules in a specific orientation; sheets of Mylar serve as a barrier to gas molecules, thereby limiting gas loss Probe a slender, thin, and sometimes flexible instrument used for measurement of physical and chemical properties, and sometimes for movement or manipulation of biological tissues; physical and chemical probes generally contain sensors that report on specific properties of interest, such as temperature or ph. Regulator a device that controls the flow of a fluid 8