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CH Jol-Principles of Chemistry I

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CH Jol-Principles of Chemistry I 1 DETERMINATION OF THE MOLAR MASS OF CARBON DIOXIDE INTRODUCTION Molar masses of gases can be determined in a number of ways, many of them indirect. In this experiment you will find the molar mass directly, using a measured mass of a dry CO2 sample, its volume, its temperature, its pressure, and the Ideal Gas Law. You will generate your CO2 sample from the action of hydrochloric acid on marble chips, a form of calcium carbonate. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g) You will do this in the apparatus shown below. CaC12 drying tube Fla sk 1 Fla sk 2 Figure 1 Carbon dioxide will be generated in flask 1 from the addition of 6M hydrochloric acid to marble chips covered with water in the bottom of the flask. The continuously generated gas will be forced from the flask into a drying tube filled with anhydrous calcium chloride pellets which will absorb any water carried by the CO2. The dry CO2 leaves the drying tube and flows through flask 2, flushing out air and carbon dioxide as it does so. The arrows on the apparatus show the path of CO2 flow. The flow of CO2 through flask 2 will be continued for 10 minutes in order to make sure that there is only CO2 in flask 2. At this point, you will collect the data needed to calculate the molar mass of the gas as follows: 2 Pressure of the Gas Sample (P) Since the gas is being generated in a generator open to the atmosphere, the pressure of the gas sample will be the same as the atmospheric pressure measured with a barometer. Temperature of the Gas Sample (T) The gas temperature will be determined in degrees Celsius using a conventional thermometer. This temperature will be converted to Kelvins for use in the Ideal Gas Law. Volume of the Gas Sample (V) Since gas samples always fill their containers, the volume of the gas sample will equal the volume of flask 2 plus the volume of its stopper assembly (stopper and both glass tubes). You will determine the volume of the flask-stopper system by filling the flask-stopper system with water and weighing it. By subtracting the mass of the flask-stopper system filled with air from the flask-stopper system filled with water, you obtain the mass of the water in the system. (Actually this subtraction gives you the mass of water in the flask minus the mass of the air included in the mass of flask-stopper system. However, the mass of the water filling the flask is so much greater than the mass of air that previously filled the flask, that the mass of the water minus the mass of the air is essentially equal to the mass of the water. The mass of the air filling the flask is essentially negligible when subtracted from the mass of the water filling the flask.) The density of liquid water varies with temperature and has been tabulated in many reference sources, but has a value of 1.000 g/mL up to a temperature of 23.1oC. The volume of the water can be determined from the mass of the water and the density of water at the experimental temperature. The volume of the gas sample is equal to this volume of the water. Mass of the Gas Sample (m) You will obtain the mass of the CO2 sample by subtracting the mass of the empty flask- stopper system from the mass of the flask-stopper system filled with CO2. The mass of the empty stopper-flask system is a little tricky to find, since all of the masss of the stopper-flask system have included mass of water, air or CO2. You will have to find the mass of the empty flask indirectly. One way to do this is to find the mass of the air in the flask-stopper system when you weighed it and subtract it from the mass of the flask-stopper system filled with air. To find this mass of air, you will take advantage of the relationship between density, volume and mass. mass density= (Eq. 1) volume When utilizing this relationship for a gas, both the volume and the density of the gas sample must be known at the same temperature and pressure. You will know the volume of air at a nonstandard T and P and the density of air at standard T and P (1.292 g/L at STP). You will have to convert one of these parameters (density or volume of air) to the conditions of the other parameter to find the correct mass of the air in the flask-stopper system. Subtraction of the mass of air from the mass of the flask-stopper system filled with air gives the mass of the 3 empty flask-stopper system. Subtraction of this mass from the mass of the flask-stopper system filled with CO2 gives you the mass of the CO2 sample collected. Using one commonly used forms of the Ideal Gas Law, it should be a simple task to calculate the molar mass of CO2: M = mRT (Eq. 2) PV where M is molar mass, m is mass in grams, and the other symbols have their usual significance. PROCEDURE The apparatus you will use is sketched on the first page of this handout. Flask 2 is a clean and dry 125-mL Erlenmeyer flask equipped with a 2-hole rubber stopper containing two tubes bent at right angles. One of these tubes extends to the bottom of flask 2, the other extends to just below the bottom of the stopper. Attach a drying tube, filled with chunks of anhydrous calcium chloride held loosely in place by plugs of cotton, to a ring stand. Connect the bulb end of this drying tube to the longer bent tube in flask 2, using a short piece of rubber tubing. Then attach the shorter of flask 2’s bent tubes to the vacuum connection in the lab. This will pull air through the drying tube and through flask 2. Let this continue for two minutes. The purpose of this step is to make certain that flask 2 is filled with dry, room temperature air. While this is being done, prepare generator flask 1 using a second 125-mL flask as the generator, and add approximately 30 grams of marble chunks. Insert a 2- hole rubber stopper containing a thistle tube and a short, right angle tube. The thistle tube must extend to near the bottom of the flask. Add approximately 20 mL water to the flask so that the bottom of the thistle tube is covered. The generator flask 1 is now ready for the experiment. By this time, flask 2 is ready as well. Disconnect flask 2 from the vacuum and from the drying tube. Immediately, to prevent any moist lab air from entering the flask, plug the two tubes in flask 2's stopper with rolled up plugs of plastic film (Parafilm). Weigh flask 2, its stopper, glass tubes and two plastic film plugs to the nearest milligram. (Keep the plastic plugs for subsequent weighings). From this point on, do not use your hands to handle flask 2. Handle with crucible tongs, so as not to add the mass of oils and crud from your fingers to this accurately known mass. Record this mass in the Data and Calculation Sheet. Attach generator flask 1 to the drying tube to flask 2 as shown in the diagram. Be sure that all stoppers are tight. Ask your instructor to check your apparatus before going on. Begin to add 6M HCl to the generator flask through the thistle tube. A 10 mL portion is sufficient for a start. Foaming indicates that CO2 is being formed. You must not allow the generator to stop producing CO2 because you do not want any air to diffuse back into flask 2 through the open tube on flask 2's stopper. You can verify that CO2 is still being produced by covering the open tube on flask 2 with your finger. If CO2 is being generated, liquid will rise up the thistle tube. Check frequently for CO2 generation, and add more 6M HCl as needed. After generating dry CO2 and passing it through flask 2 for at least 15 minutes, disconnect flask 2 and plug the two tubes with your plastic film plugs. Weigh 4 flask 2, its stopper assembly and its CO2 to the nearest milligram and record this in your notebook. After removing the plastic plugs, reattach flask 2 to the generator flask, and generate dry CO2 for another 10 minutes. At the end of this time, again disconnect flask 2, replug its tubes and weigh it again. The two successive CO2 flask weighings should differ by no more than 4 mg (0.004g). If they differ by more than 4 mg, generate more CO2 through flask 2 until two successive weighings of flask 2 filled with CO2 differ by no more than 4 mg. At this point remove the stopper from flask 2 and measure the temperature of the CO2. Also obtain the barometric pressure for the day. Record these data in the Data and Calculation Sheet. Finally, fill flask 2 to the brim with water. Insert the stopper with its glass tubes tightly into the flask. With an eyedropper fill the glass tubes with water and plug them. There should be no air bubbles anywhere in your apparatus. Weigh flask 2 and its stopper full of water and record this mass in the Data and Calculation Sheet.
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